Semiconductor light emitting device and method for fabricating the same

ABSTRACT

A semiconductor light emitting device is composed of a blue light emitting diode, a red light emitting layer grown epitaxially on the blue light emitting diode, and an insulating material containing a YAG fluorescent material. The red light emitting layer is made of, e.g., undoped In 0.4 Ga 0.6 N having a forbidden band width of 1.9 eV and formed on a p-type semiconductor layer to have a configuration consisting of a plurality of mutually spaced-apart islands.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/060,425,filed Feb. 18, 2005, which is incorporated by reference herein in itsentirety.

The teachings of Japanese Patent Applications JP 2004-42329 and JP2004-42330, each filed Feb. 19, 2004, are entirely incorporated hereinby reference, inclusive of the claims, specification, and drawings.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light emitting devicewhich is applicable to, e.g., a white light emitting diode and to amethod for fabricating the same.

A gallium nitride-based (GaN-based) group III-V nitride semiconductor(InGaAlN) in which GaN has a large forbidden bandwidth of, e.g., 3.4 eVat a room temperature is a material which can implement a light emittingdevice capable of a high output in the blue or green wavelength band oreven in the ultraviolet wavelength band. The GaN-based semiconductor hasalready been commercialized as a blue/green light emitting diode invarious display panels, large-scale display devices, and trafficsignals.

On the other hand, a white light emitting diode which provides whitelight by exciting a YAG fluorescent material with emission light from ablue light emitting diode has also been commercialized already forvarious applications including the back light of a liquid crystaldisplay panel.

If the white light emitting diode can be enhanced in brightness andlight emitting efficiency, a semiconductor illuminating device as areplacement for currently prevailing fluorescent and incandescent lampscan be provided. Accordingly, white light emitting diodes forillumination are predicted to create an extremely large market in thefuture.

For illumination purposes, the improvement of the manner in which colorappears when a white light emitting diode is used for illumination,i.e., a color rendering property is important in addition to theenhancement of brightness and the light emitting efficiency.

Since each of the white light emitting diodes that have beencommercialized heretofore has used a method which excites a YAG (YttriumAluminum Garnet) fluorescent material with emission light at about 470nm from a blue light emitting diode and thereby obtains yellow emissionlight (see, e.g., S. Nakamura et al., “The Blue Laser Diode”Springer-Verlag Berlin Heidelberg N.Y.: See p. 216), the problem hasbeen encountered that the amount of red emission light in an emissionspectrum is small and the color rendering property is consequentlyinferior to that of light from a fluorescent lamp and light from anincandescent lamp.

At present, there is no red fluorescent material that exhibits asufficiently high excitation efficiency upon excitation caused by blueemission light. To improve the color rendering property, therefore, itis particularly necessary to enhance the brightness of a red lightemitting fluorescent material.

Referring to FIG. 36, a description will be given herein below to astructure of a conventional white light emitting diode which provideswhite light by mixing blue light from a GaN-based blue light emittingdiode with yellow light which is outputted through the excitation of aYAG fluorescent material with the blue light and to the light emissioncharacteristics thereof.

As shown in FIG. 36, the white light emitting diode is composed of ablue light emitting diode 300 for outputting blue light at a wavelengthof 470 nm which has been mounted on a package 310 to have upper and sidesurfaces thereof covered with an insulating material 320 containing aYAG fluorescent material.

A method for fabricating the blue light emitting diode 300 will bedescribed, in which an n-type GaN layer 302, an active layer 303 made ofInGaN, and a p-type GaN layer 304 are formed successively on a substrate301 made of sapphire by, e.g., MOCVD

(Metal Organic Chemical Vapor Deposition),

Next, dry etching using, e.g., a chlorine gas is performed with respectto the n-type GaN layer 304 and the active layer 303, therebyselectively exposing a part of the n-type GaN layer 302. Subsequently,an n-side electrode 305 composed of a multilayer film of titanium andgold is formed on the exposed portion of the n-type GaN layer 302. Onthe other hand, a p-side transparent electrode 306 composed of amultilayer film of nickel and gold and having a reduced thickness of 10nm or less for the transmission of light emitted from the active layer303 is formed on the p-type GaN layer 304.

Next, a pad electrode 307 made of gold is formed selectively on thep-side transparent electrode 306, whereby a majority of the blue lightemitted from the active layer 303 passes through the p-side transparentelectrode 306.

Next, the blue light emitting diodes 300 in the form of a wafer isdivided into square chips each having, e.g., 350-μm sides. Each of thechips is mounted at a specified position on the package 310 and thenwire bonding is performed with respect to the n-side electrode and thepad electrode 307. Subsequently, the insulting material 320 containingthe YAG fluorescent material is coated or applied dropwise to cover theblue light emitting diode 300 and then hardened.

The light emission characteristics of the white light emitting diodethus obtained are as shown in FIG. 37 so that white light which is amixture of blue emission light 300A from the blue light emitting diode300 and yellow emission light 320A from the YAG fluorescent material 320is emitted.

However, the conventional white light emitting diode has the problem ofa poor white color rendering property, as shown in FIG. 37, due to thesmall amount of the red emission light component in a spectrum obtainedfrom the blue light emitting diode 300.

SUMMARY OF THE INVENTION

In view of the conventional problems described above, it is therefore anobject of the present invention to provide a white light emitting diodemade of a GaN-based compound semiconductor having an excellent colorrendering property.

To attain the object, the present invention provides a structure whereina light emitting diode for emitting short-wavelength light such as bluelight or ultraviolet light has a semiconductor layer which is excited bylight emitted from the light emitting diode to release long-wavelengthlight such as green light or red light.

The present invention also provides another structure wherein anothern-type semiconductor layer is provided to come in contact with a p-typesemiconductor layer composing the light emitting diode in conjunctionwith an n-type semiconductor layer such that a tunnel current is allowedto flow between the other n-type semiconductor layer and the p-typesemiconductor layer. The arrangement obviates the necessity for atransparent electrode used for the light emitting diode.

Specifically, a first semiconductor light emitting device according tothe present invention comprises: a light emitting diode for emittingfirst emission light; and a semiconductor film provided in a portion ofthe light emitting diode to absorb the first emission light and emitsecond emission light, wherein the semiconductor film emits the secondemission light through optical excitation by the first emission light.

The first semiconductor light emitting device allows the light emittingdiode to output blue light as the first emission light and allows thesemiconductor film to absorb the blue first emission light and releasethe second emission light, which is either red light or green light, sothat the white light emitting diode having an excellent color renderingproperty is provided.

Preferably, the first semiconductor light emitting device furthercomprises: a transparent electrode provided in the light emitting diode,wherein the transparent electrode transmits the first emission light orthe second emission light. The arrangement enhances the efficiency withwhich a current is injected in the light emitting diode to cause lightemission from the diode.

In the first semiconductor light emitting device, the semiconductor filmis preferably formed on a part of a light emitting surface of thesemiconductor light emitting device. In the arrangement, a part of thefirst emission light is absorbed by the semiconductor film so that thereis no conversion of the remaining part of the first emission light tothe second emission light. This ensures the emission of white lighthaving an excellent color rendering property.

Preferably, the first semiconductor light emitting device furthercomprises: a fluorescent material covering the light emitting diode andthe semiconductor film, wherein the fluorescent material absorbs thefirst emission light and emits third emission light. In the case where aYAG fluorescent material is used for the fluorescent material, thearrangement allows the emission of high-brightness yellow light throughexcitation caused by blue light. By combining the yellow light from thefluorescent material with the high-brightness red light from thesemiconductor film, therefore, white light having an excellent colorrendering property is obtainable.

In the first semiconductor light emitting device, the light emittingdiode or the semiconductor film is preferably formed on a substrate madeof a single crystal. The arrangement allows the formation the lightemitting diode or the semiconductor film on the substrate made of asingle crystal by epitaxial growth. As a result, the crystal property ofthe light emitting diode or the semiconductor film is improved andtherefore a high-brightness white light emitting diode can be provided.

In the first semiconductor light emitting device, the substratepreferably transmits the first emission light and the second emissionlight. The arrangement allows the extraction of the first emission lightand the second emission light through the substrate. Accordingly, in thecase where the light emitting diode and the semiconductor film areformed on the substrate in this order, e.g., so-called flip-chipmounting can be performed which forms an electrode made of a materialhaving a high reflectivity with respect to each of the first emissionlight and the second emission light on the semiconductor film and mountsthe formed electrode in opposing relation to the surface of a package.As a result, a white light emitting diode featuring an excellent heatdissipation property and high brightness can be obtained.

In the case where the transparent electrode is provided, the transparentelectrode is preferably provided with a plurality of openings and thesemiconductor film is preferably formed in each of the openings on alight emitting surface of the light emitting diode. The arrangementallows the second emission light from the semiconductor film to beextracted without passing through the transparent electrode so that areduction in brightness due to the absorption of light by thetransparent electrode no more occurs. In addition, the semiconductorfilm is in contact with the light emitting diode so that the brightnessof the second emission light is also enhanced.

In this case, the semiconductor film is preferably formed in mutuallydivided relation in each of the openings on a light emitting surface ofthe light emitting diode.

In this case, the semiconductor film is preferably formed to cover apart of the transparent electrode in the vicinity of the openings.

In the first the semiconductor light emitting device, the semiconductorfilm has a crystal defect density which is lower in the portion thereoflocated on the transparent electrode than in each of the portionsthereof located over the individual openings of the transparentelectrode.

In the case where the semiconductor film or the light emitting diode isthus formed to grow selectively in a lateral direction from each of theopenings in the transparent electrode, the crystal property of theportion of the semiconductor film or the light emitting diode located onthe transparent electrode is improved so that light emission isperformed with higher brightness.

In the first semiconductor light emitting device, the first emissionlight is preferably blue light or ultraviolet light.

In this case, the semiconductor film is preferably excited by the firstemission light to emit the second emission light which is red light. Inthe arrangement, if the semiconductor film and the light emitting diodeare covered with, e.g., an insulating material containing a YAGfluorescent material, a white light emitting diode having an excellentwhite color rendering property is obtainable.

In the first semiconductor light emitting device, the semiconductor filmis preferably composed of a plurality of semiconductor films which arestacked in layers and emit emission light components having differentwavelengths from each of the stacked layers.

In the first semiconductor light emitting device, the semiconductor filmis preferably doped with impurities and excited by the first emissionlight to emit the second emission light in a visible range via energylevels resulting from the impurities. The arrangement allowshigh-brightness light emission from the semiconductor film doped withthe impurity via the energy level of the impurity which can serve as ahigh-brightness luminescent center. As a result, the brightness of thesecond emission light can be enhanced and the color rendering propertycan further be improved.

In the first semiconductor light emitting device, the semiconductor filmis preferably doped with an impurity to emit the second emission lightin a visible range through light emission from inner-shell transition ofthe impurities. The arrangement realizes high-efficiency red lightemission and allows a white light emitting diode having a more excellentcolor rendering property to be obtained.

In this case, the impurity is preferably Eu, Sm, or Yb.

Preferably, the impurities in the semiconductor film are introduced byion implantation. The arrangement allows the semiconductor film to bedoped with the impurity with high controllability.

In the first semiconductor light emitting device, the light emittingdiode or the semiconductor layer is preferably composed of a compoundsemiconductor containing nitrogen. The arrangement ensures the emissionof high-brightness blue light or ultraviolet light as the first emissionlight and the emission of high-brightness red light as the secondemission light so that white light having an excellent color renderingproperty is obtainable.

In the first semiconductor light emitting device, the transparentelectrode is preferably composed of a conductive material containingnickel or indium tin oxide (ITO) each having a thickness more than 0 nmand not more than 20 nm. The arrangement reduces the absorption of lightemitted and absorbed by the transparent electrode. In addition, sincenickel, e.g., can form an excellent ohmic contact with a nitridesemiconductor layer, a white light emitting diode featuring highbrightness and a reduced series resistance can be provided.

In the case where an insulating material containing a fluorescentmaterial is provided, the fluorescent material is preferably yttriumaluminum garnet (YAG). The arrangement allows high-brightness yellowlight to be obtained by using the blue excitation light from the lightemitting diode. By mixing the blue emission light with the red secondemission light from the semiconductor film, e.g., a high-brightnesslight emitting diode having an excellent white color rendering propertycan be provided.

In the case where the light-emitting diode is formed on the substratemade of a single crystal, there can be used sapphire, silicon carbide,gallium nitride, aluminum nitride, magnesium oxide, lithium galliumoxide, lithium aluminum oxide, lithium aluminum oxide, or a mixedcrystal of lithium gallium oxide and lithium aluminum oxide as thesingle crystal. Since the use of such a single crystal allows a nitridesemiconductor layer having an excellent crystal property to be formed onthe substrate, higher-brightness blue or ultraviolet light can beobtained as the first emission light so that higher-brightness redsecond emission light is obtainable by using the first emission light asthe excitation light.

Preferably, the first semiconductor light emitting device furthercomprises a metal film provided in the light emitting diode and having athickness of at least 10 μm, wherein a current is injected in the lightemitting diode through the metal film. The arrangement allows heatgenerated from the light emitting diode in operation to dissipatethrough the metal film so that a white light emitting diode having anexcellent heat dissipation property and capable of high-output operationis obtainable.

In this case, the metal film is preferably made of gold, copper, orsilver.

In the first semiconductor light emitting device, the light emittingdiode is preferably provided with a metal electrode having areflectivity of 60% or more with respect to the first emission light orthe second emission light. The arrangement remarkably increases theefficiency with which emission light is extracted.

In this case, the metal electrode is preferably composed of asingle-layer film or a multi-layer film each made of at least onematerial selected from the group consisting of gold, platinum, copper,silver, and rhodium.

A first method for fabricating a semiconductor light emitting deviceaccording to the present invention comprises the steps of: (a) forming alight emitting diode composed of a plurality of semiconductor layers ona substrate made of a single crystal; and (b) selectively forming, onthe light emitting diode, a semiconductor film which absorbs firstemission light and emits second emission light through the opticalexcitation by the first emission light.

Since the first method for fabricating a semiconductor light emittingdevice allows the light emitting diode formed on the substrate made ofthe single crystal to emit blue light as the first emission light andallows the semiconductor film formed selectively on the light emittingdiode to absorb the blue first emission light and release the secondemission light which is either red or green, a white light emittingdiode having an excellent color rendering property can be provided.

Preferably, the first method for fabricating a semiconductor lightemitting device further comprises the step of: (c) between the steps (a)and (b) or after the step (b), forming a transparent electrode whichtransmits the first emission light on the light emitting diode.

In this case, the step (c) preferably includes the step of providing aplurality of openings in the transparent electrode. The arrangementallows the second emission light released from the semiconductor film tobe extracted without passing through the transparent electrode so that abrightness reduction due to the absorption of light by the transparentelectrode no more occurs. As a result, a high-brightness white lightemitting diode can be obtained.

In this case, the semiconductor film is preferably formed in the step(b) to selectively grow from each of the regions of an upper surface ofthe light emitting diode which are exposed through the plurality ofopenings of the transparent electrode.

Preferably, the first method for fabricating a semiconductor lightemitting device further comprises the step of: (d) after the step (b),irradiating the surface of the substrate opposite to the light emittingdiode with light having a wavelength which is not absorbed by thesubstrate but is absorbed by the semiconductor layers composing thelight emitting diode to separate the substrate from the diode. Thearrangement allows a white light emitting diode having an excellent heatdissipation property and capable of large-output operation to beprovided through the separation of the substrate when the substrate ismade of a material having a poor heat dissipation property such as,e.g., sapphire.

In this case, the step (d) preferably includes forming, in a part of thesemiconductor layers irradiated with the light, a decomposition layerresulting from thermal decomposition of the irradiated semiconductorlayers. Even when the substrate has a relatively larger area, thearrangement allows no-split separation of the substrate from the lightemitting diode with high reproducibility.

A second method for fabricating a semiconductor light emitting deviceaccording to the present invention comprises the steps of: (a) forming asemiconductor film on a substrate made of a single crystal; and (b)forming a light emitting diode composed of a plurality of semiconductorlayers on the semiconductor film, wherein the semiconductor film absorbsfirst emission light emitted from the light emitting diode and emitssecond emission light through the optical excitation by the firstemission light.

Since the second method for fabricating a semiconductor light emittingdevice allows the semiconductor film and the light emitting diode to beformed on the substrate made of the single crystal, allows the lightemitting diode to output, e.g., blue light as first emission light, andallows the semiconductor film to absorb the blue first emission lightand release the second emission light, which is either red or green, awhite light emitting diode having an excellent white color renderingproperty can be provided.

Preferably, the second method for fabricating a semiconductor lightemitting device further comprises the step of: (c) between the steps (a)and (b), forming a transparent electrode which transmits the firstemission light on the semiconductor film.

In this case, the step (c) preferably includes the step of providing aplurality of openings in the transparent electrode.

In this case, a lower one of the plurality of semiconductor layers ispreferably formed in the step (b) to selectively grow from each of theregions of an upper surface of the semiconductor film which are exposedthrough the plurality of openings of the transparent electrode.

Preferably, the second method for fabricating a semiconductor lightemitting device further comprises the step of: (d) after the step (b),irradiating the surface of the substrate opposite to the semiconductorfilm with light having a wavelength which is not absorbed by thesubstrate but is absorbed by the semiconductor film to separate thesubstrate from the semiconductor film.

In this case, the step (d) preferably includes forming, in a part of thesemiconductor layers irradiated with the light, a decomposition layerresulting from thermal decomposition of the irradiated semiconductorlayers.

Preferably, the first or second method for fabricating a semiconductorlight emitting device further comprises the step of: (e) after the step(d), forming a metal film on the light emitting diode or on thesemiconductor film.

A second semiconductor light emitting device according to the presentinvention comprises: a light emitting diode composed of a first n-typesemiconductor layer, a p-type semiconductor layer, and a second n-typesemiconductor layer which are formed successively to emit first emissionlight; and a semiconductor film provided in the light emitting diode toabsorb the first emission light and emit second emission light throughthe optical excitation by the first emission light, wherein when avoltage is applied such that the second n-type semiconductor layer has apositive potential relative to the first n-type semiconductor layer, acurrent flows from the second n-type semiconductor layer to the firstn-type semiconductor layer.

Since the second semiconductor light emitting device allows the lightemitting diode to output blue light as the first emission light andallows the semiconductor film to absorb the blue first emission lightand release the second emission light, which is either red or green, awhite light emitting diode having an excellent white color renderingproperty can be provided. In addition, when an operating current isinjected in the light emitting diode, a structure is adopted in which aso-called tunnel current flows in a p⁺n⁺ junction composed of the p-typesemiconductor layer and the second n-type semiconductor layer so that arectifying property is obtained by applying a voltage between the secondn-type semiconductor layer and the first n-type semiconductor layer suchthat it is higher to the second n-type semiconductor layer than to thefirst n-type semiconductor layer. Since the heavily doped second n-typesemiconductor layer has a low resistance, a current is more likely to bedispersed in the in-plane direction of a pn junction surface. Thisobviates the necessity to use the transparent electrode and enhances thebrightness of the emission light.

In the second semiconductor light emitting device, an impurityconcentration in preferably each of the p-type semiconductor layer andthe second n-type semiconductor layer is 10¹⁸ cm³ or more. This morepositively allows a tunnel current to flow in the p⁺n⁺ junction betweenthe p-type semiconductor layer and the second n-type semiconductorlayer.

Preferably, the second semiconductor light emitting device furthercomprises: a fluorescent material covering the light emitting diode andthe semiconductor film which absorbs the first emission light and emitsthird emission light through the optical excitation by the firstemission light.

In the case where a YAG fluorescent material, e.g., is used for afluorescent material, the arrangement allows high-brightness yellowlight to be emitted as the third emission light through excitationcaused by blue light. As a result, a white light emitting diode havingan excellent color rendering property can be provided by combining theyellow third emission light with the high-brightness second emissionlight which is red.

In the second semiconductor light emitting device, the light emittingdiode or the semiconductor film is preferably formed on a substrate madeof a single crystal. Since the arrangement allows the light emittingdiode and the semiconductor film to be formed on the substrate made ofthe single crystal by epitaxial growth, the crystal property of each ofthe first n-type semiconductor layer, the p-type semiconductor layer,the second n-type semiconductor layer, and the semiconductor film isimproved so that a high-brightness white light emitting diode isobtainable. In addition, since the second emission light released fromthe semiconductor film can be extracted through the substrate, flip-chipmounting which forms, e.g., a high-reflectivity electrode on thesemiconductor film can be performed so that a white light emitting diodehaving an excellent heat dissipation property is provided.

Preferably, the second semiconductor light emitting device furthercomprises: a metal film provided on the light emitting diode to have athickness of at least 10 μm, wherein an operating current is injected inthe light emitting diode through the metal film. The arrangement allowsheat generated from the light emitting diode in operation to dissipatethrough the metal film so that a white light emitting diode having anexcellent heat dissipation property and capable of high-output operationis obtainable.

In this case, the metal film is preferably made of gold, copper, orsilver.

In the second semiconductor light emitting device, the light emittingdiode is preferably provided with a metal electrode having areflectivity of 60% or more with respect to the first emission light orthe second emission light. In the arrangement, the first or secondemission light is reflected strongly by the metal electrode so that theefficiency with which emission light is extracted is remarkablyimproved.

In this case, the metal electrode is preferably composed of asingle-layer film or a multi-layer film each made of at least onematerial selected from the group consisting of gold, platinum, copper,silver, and rhodium.

In the second semiconductor light emitting device, the first emissionlight is preferably blue light or ultraviolet light. The arrangementallows a white light emitting diode having an excellent white colorrendering property to be obtained by, e.g., covering the light emittingdiode and the semiconductor film with an insulating material containinga YAG fluorescent material.

In the second semiconductor light emitting device, the second emissionlight is preferably red light. The arrangement provides a white lightemitting diode having an excellent white color rendering property bycovering the light emitting diode and the semiconductor film with aninsulating material which contains a YAG fluorescent material and emitsblue light as the first emission light.

In the second semiconductor light emitting device, the semiconductorfilm is preferably composed of a plurality of semiconductor films whichare stacked in layers and emit emission light components havingdifferent wavelengths from each of the stacked layers. The arrangementallows a white light emitting diode having an excellent white colorrendering property to be provided by constituting the plurality ofsemiconductor films such that they emit light in three colors which arered, green, and blue through excitation caused by, e.g., ultravioletlight.

In the second semiconductor light emitting device, the semiconductorfilm is preferably doped with impurities and the semiconductor film ispreferably excited by the first emission light to emit the secondemission light in a visible range via energy levels resulting from theimpurities. The arrangement provides high-brightness emission light viathe energy level of the impurity which may serve as a high-brightnessluminescent center in the semiconductor film doped with the impurity.Accordingly, the brightness of the second emission light can be enhancedand the color rendering property can further be improved.

In the second semiconductor light emitting device, the semiconductorfilm is preferably doped with impurities to emit the second emissionlight in a visible range through light emission from inner-shelltransition of the impurities. The arrangement allows high-efficiency redlight emission and provides a white light emitting diode having a moreexcellent color rendering property.

In this case, the impurity is preferably Eu, Sm, or Yb.

Preferably, the impurities of the semiconductor film in the impurity areintroduced by ion implantation. The arrangement allows the semiconductorfilm to be doped with the impurity with high controllability.

In the second semiconductor light emitting device, the light emittingdiode or the semiconductor film is preferably made of a compoundsemiconductor containing nitrogen. The arrangement ensures the emissionof high-brightness blue light or ultraviolet light as the first emissionlight and the emission of high-brightness red light as the secondemission light so that white light having an excellent color renderingproperty is obtainable.

In the case where the light emitting diode and the semiconductor filmare covered with an insulating material containing a fluorescentmaterial, the fluorescent material is preferably yttrium aluminum garnet(YAG). The arrangement allows high-brightness yellow light to beobtained by using the blue excitation light from the light emittingdiode. By mixing the blue emission light with the red second emissionlight from the semiconductor film, e.g., a high-brightness lightemitting diode having an excellent white color rendering property can beprovided.

In the case where the light emitting diode or the semiconductor film isformed on the substrate made of a single crystal, the single crystal ispreferably sapphire, silicon carbide, gallium nitride, aluminum nitride,magnesium oxide, lithium gallium oxide, lithium aluminum oxide, or amixed crystal of lithium gallium oxide and lithium aluminum oxide. Sincethe use of such a single crystal allows a nitride semiconductor layerhaving an excellent crystal property to be formed on the substrate,higher-brightness blue or ultraviolet light can be obtained as the firstemission light so that higher-brightness red second emission light isobtainable by using the first emission light as the excitation light.

A third method for fabricating a semiconductor light emitting deviceaccording to the present invention comprises the steps of: (a)successively depositing a first n-type semiconductor layer, a p-typesemiconductor layer, and a second n-type semiconductor layer on asubstrate made of a single crystal to form a light emitting diode; and(b) selectively forming, on the light emitting diode, a semiconductorfilm which absorbs first emission light emitted from the light emittingdiode and emit second emission light through the optical excitation bythe first emission light, wherein an impurity concentration in each ofthe p-type semiconductor layer and the second n-type semiconductor layeris set in the step (a) to allow a current to flow from the second n-typesemiconductor layer to the first n-type semiconductor layer when avoltage is applied such that the second n-type semiconductor layer has apositive potential relative to the first n-type semiconductor layer.

Since the third method for fabricating a semiconductor light emittingdevice allows the light emitting diode to output blue light as the firstemission light and allows the semiconductor film to absorb the bluefirst emission light and release the second emission light, which iseither red or green, a white light emitting diode having an excellentwhite color rendering property can be provided. In addition, an impurityconcentration in each of the p-type semiconductor layer and the secondn-type semiconductor layer is set to allow a tunnel current to flow inthe p⁺n⁺ junction surface therebetween so that a rectifying property isobtained by applying a voltage between the second n-type semiconductorlayer and the first n-type semiconductor layer such that it is higher tothe second n-type semiconductor layer than to the first n-typesemiconductor layer. Since the heavily doped second n-type semiconductorlayer has a low resistance, a current is more likely to be dispersed inthe in-plane direction of the pn junction surface. This obviates thenecessity to use the transparent electrode and enhances the brightnessof the emission light.

In the third method for fabricating a semiconductor light emittingdevice, the impurity concentration in each of the p-type semiconductorlayer and the second n-type semiconductor layer is preferably 10¹⁸ cm³or more.

Preferably, the third method for fabricating a semiconductor lightemitting device further comprises the step of: (c) after the step (b),separating the substrate from the light emitting diode. The arrangementallows a white light emitting diode having an excellent heat dissipationproperty and capable of large-output operation to be provided throughthe separation of the substrate when the substrate is made of a materialhaving a poor heat dissipation property such as, e.g., sapphire.

In this case, the step (c) preferably includes the step of irradiatingthe surface of the substrate opposite to the light emitting diode withlight having a wavelength which is not absorbed by the substrate but isabsorbed by the semiconductor layers composing the light emitting diodeto form, in a part of the semiconductor layers irradiated with thelight, a decomposition layer resulting from decomposition of thesemiconductor layers. Even when the substrate has a relatively largerarea, the arrangement allows no-split separation of the substrate fromthe light emitting diode with high reproducibility.

In this case, the third method for fabricating a semiconductor lightemitting device preferably further comprises the steps of: (d) prior tothe step (c), bonding a holding substrate made of a material other thanthat of the substrate to the light emitting diode or to thesemiconductor film; and (e) after the step (c), separating the holdingsubstrate from the light emitting diode or from the semiconductor film.The arrangement suppresses the occurrence of a crack in thesemiconductor film in the process in which stress in the semiconductoris reduced by thermal decomposition during the formation thedecomposition layer in the step (c). Accordingly, even when thesubstrate having a large area is used, the substrate can be separatedwithout causing a crack in the light emitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a semiconductor light emitting device according toa first embodiment of the present invention, of which FIG. 1A is across-sectional view thereof and FIG. 1B is a plan view thereof;

FIG. 2 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the first embodiment;

FIGS. 3A through 3C are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the first embodiment;

FIGS. 4A through 4C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the first embodiment;

FIG. 5 is a cross-sectional view showing a semiconductor light emittingdevice according to a second embodiment of the present invention;

FIGS. 6A and 6B show a semiconductor light emitting device according toa third embodiment of the present invention, of which FIG. 6A is across-sectional view thereof and FIG. 6B is a plan view thereof;

FIGS. 7A through 7D are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the third embodiment;

FIGS. 8A through 8C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the third embodiment;

FIGS. 9A and 9B show a semiconductor light emitting device according toa fourth embodiment of the present invention, of which FIG. 9A is across-sectional view thereof and FIG. 9B is a plan view thereof;

FIG. 10 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the fourth embodiment;

FIGS. 11A through 11C are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the fourth embodiment;

FIGS. 12A and 12B are cross-sectional views illustrating the individualprocess steps of the method for fabricating the semiconductor lightemitting device according to the fourth embodiment;

FIGS. 13A and 13B show a semiconductor light emitting device accordingto a fifth embodiment of the present invention, of which FIG. 13A is across-sectional view thereof and FIG. 13B is a plan view thereof;

FIG. 14 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the fifth embodiment;

FIGS. 15A through 15E are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the fifth embodiment;

FIGS. 16A through 16C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the fifth embodiment;

FIGS. 17A and 17B show a semiconductor light emitting device accordingto a sixth embodiment of the present invention, of which FIG. 17A is across-sectional view thereof and FIG. 17B is a plan view thereof;

FIGS. 18A through 18D are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the sixth embodiment;

FIGS. 19A through 19C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the sixth embodiment;

FIG. 20 is a cross-sectional view showing a semiconductor light emittingdevice according to a seventh embodiment of the present invention;

FIG. 21 is a plan view showing the semiconductor light emitting deviceaccording to the seventh embodiment;

FIG. 22 is a graph showing the concentration profile of Eu ions used todope a red light emitting Eu doped layer in the semiconductor lightemitting device according to the seventh embodiment;

FIG. 23 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the seventh embodiment;

FIGS. 24A through 24D are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the seventh embodiment;

FIGS. 25A through 25C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the seventh embodiment;

FIGS. 26A and 26B show a semiconductor light emitting device accordingto an eighth embodiment of the present invention, of which FIG. 26A is across-sectional view thereof and FIG. 26B is a plan view thereof;

FIG. 27A is a graph showing voltage-current characteristics in thepresence and absence of a tunnel junction and FIG. 27B is a graphshowing current-light output characteristics in the presence and absenceof a tunnel junction;

FIG. 28 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the eighth embodiment;

FIGS. 29A through 29E are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the eighth embodiment;

FIGS. 30A and 30B show a semiconductor light emitting device accordingto a ninth embodiment of the present invention, of which FIG. 30A is across-sectional view thereof and FIG. 30B is a plan view thereof;

FIG. 31 is a graph showing an emission spectrum obtained from thesemiconductor light emitting device according to the ninth embodiment;

FIGS. 32A through 32D are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the ninth embodiment;

FIGS. 33A and 33B show a semiconductor light emitting device accordingto a tenth embodiment of the present invention, of which FIG. 33A is across-sectional view thereof and FIG. 33B is a plan view thereof;

FIGS. 34A through 34D are cross-sectional views illustrating theindividual process steps of a method for fabricating the semiconductorlight emitting device according to the tenth embodiment;

FIGS. 35A through 35C are cross-sectional views illustrating theindividual process steps of the method for fabricating the semiconductorlight emitting device according to the tenth embodiment;

FIG. 36 is a cross-sectional view showing a conventional white lightemitting diode; and

FIG. 37 is a graph showing an emission spectrum obtained from theconventional white light emitting diode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A first embodiment of the present invention will be described withreference to the drawings.

FIG. 1A shows a cross-sectional structure of a semiconductor lightemitting device according to the first embodiment and FIG. 1B shows aplan structure thereof.

As shown in FIGS. 1A and 1B, a semiconductor light emitting deviceaccording to the first embodiment is composed of: a blue light emittingdiode 10 mounted at a specified position on a package 20 as a mountingmember; a red light emitting layer 15 grown epitaxially on the bluelight emitting diode 10; and an insulating material 19 containing anyttrium aluminum garnet (YAG) fluorescent material. In FIG. 1B, thedepiction of the insulating material 19 is omitted.

The blue light emitting diode 10 is composed of: an n-type semiconductorlayer 12 made of n-type GaN; an active layer 13 having a multiplequantum well structure made of InGaN; and a p-type semiconductor layer14 made of p-type Al_(0.05)Ga_(0.95)N which are epitaxially grownsuccessively on a substrate 11 made of, e.g., sapphire. The active layer13 is composed of three well layers each made of In_(0.35)Ga_(0.65)Nhaving a thickness of 2 nm and three barrier layers each made ofIn_(0.02)Ga_(0.98)N having a thickness of 10 nm which are alternatelystacked, thereby emitting blue light at 470 nm.

The red light emitting layer 15 is made of, e.g., undopedIn_(0.4)Ga_(0.6)N with a forbidden band width of 1.9 eV and formed onthe p-type semiconductor layer 14 to have a matrix configuration ofdiscrete and spaced-apart squares with 2-μm to 20-μm sides when viewedin a plan view. Each square of the red light emitting layer 15 isexcited by light outputted from the blue light emitting diode 10 to emitred light at 650 nm. Red light can be obtained from the red lightemitting layer 15 by doping the red light emitting layer 15 with, e.g.,zinc (Zn) and thereby reducing the composition of In, magnesium (Mg), orsilicon (Si). By thus reducing the composition of In, a lattice mismatchwith an underlie layer made of GaN normally used can be suppressed andcrystal defects in the red light emitting layer 15 can be reduced sothat high-brightness light emission is enabled. At this time, theemission light released from the red light emitting layer 15 is mixedwith excitation light in a visible range which is generated via anenergy level resulting from the impurity used for doping.

A transparent electrode 16 made of ITO (Indium Tin Oxide) has beenformed over the entire surface of the p-type semiconductor layer 14including the red light emitting layer 15. In addition, a p-sideelectrode 17 made of gold (Au) has been formed selectively on a regionof the p-type semiconductor layer 14 via the transparent electrode 16.

The n-type semiconductor layer 12 has a portion thereof exposed and ann-side electrode 18 composed of a multilayer film of titanium (Ti) andgold (Au) has been formed on the exposed region.

It is to be noted that required metal fine lines are connected by wirebonding to the p-side and n-side electrodes 17 and 18, thought they arenot depicted.

The insulating material 19 containing the YAG fluorescent material hasbeen formed in such a manner that it is coated or applied dropwise ontothe package 20 to cover the blue light emitting diode 10, thetransparent electrode 16, the p-side electrode 17, and the n-sideelectrode 18. The insulating material 19 is excited by the blue lightoutputted from the blue light emitting diode 10 and emits yellow light.

Film forming conditions for the transparent electrode 16 have beenoptimized such that the transparent electrode 16 does not absorb theblue light from the blue light emitting diode 10 and a contactresistance with the p-type semiconductor layer 14 becomes, e.g., 1×10⁻³Ωcm² or less.

A current is injected into the blue light emitting diode 10 via thep-side electrode 17, the transparent electrode 16, and the p-typesemiconductor layer 14. The blue light emitting diode 10 is capable ofoperating at a relatively low voltage of, e.g., about 3 V. Accordingly,white light having the spectrum shown in FIG. 2 can be obtained byinjecting a current of, e.g., 20 mA into the blue light emitting diode10 and thereby causing the emission of blue light at a wavelength of 470nm. In FIG. 2, the emission spectrum is composed of the transmittedcomponent 10A of the blue light at a wavelength of 470 nm, yellow light19A with a peak wavelength of 550 nm from the YAG fluorescent material,and red light 15A at a wavelength of 650 nm from the red light emittinglayer 15. The blue light 10A, the yellow light 19A, and the red light15A are mixed to provide white light.

Thus, the first embodiment allows one-chip integration of a lightemitting diode in which the red light emitting layer 15 which receivesblue light outputted from the blue light emitting diode 10 and emits redlight through excitation caused thereby is provided between theinsulating material 19 containing the YAG fluorescent material whichemits yellow light and the blue light emitting diode 10 which emits bluelight. Accordingly, the intensity of emission light in the red range ishigher than in an emission spectrum obtained from the conventional whitelight emitting diode shown in FIG. 23 which provides white light byexciting the YAG fluorescent material with blue light from the bluelight emitting diode. This allows a white light emitting diode whichoutputs white light having an excellent color rendering property to beprovided.

The blue light emitting diode 10 may also be formed with an underlielayer made of GaN and a thin-film buffer layer made of GaN or AlN beinginterposed between the substrate 11 made of sapphire and the n-typesemiconductor layer 12.

The active layer 13 may also be constituted to have the composition ofIn which is nonuniform in the in-plane direction (direction parallel toa substrate surface) of the active layer 13.

Instead of varying a lattice constant in each of the n-typesemiconductor layer 12, the active layer 13, the p-type semiconductorlayer 14, and the read semiconductor layer 15 which have been formed onthe substrate 11, the composition of a group III element in a quaternaryor higher-order mixed crystal may also be varied in forming the bluelight emitting diode 10 and the red light emitting layer 15. Thisprovides a structure from which high-brightness light emission can beobtained without incurring a crystal defect due to a lattice mismatchand the resulting nonradiative recombination.

In the first embodiment, the YAG fluorescent material and the red lightemitting layer 15 are excited by the output light received thereby fromthe blue light emitting diode 10 and emit yellow light and red light,respectively, thereby providing white light. However, an ultravioletlight emitting diode which output ultraviolet light at a wavelength of,e.g., 340 nm may also be formed in place of the blue light emittingdiode 10. In this case, a blue light emitting fluorescent material and agreen light emitting fluorescent material are added to the insulatingmaterial 19.

It is also possible to separate the substrate 11 made of sapphire fromthe blue light emitting diode 10 and provide a metal film in place ofthe separated substrate. The arrangement allows the use of the providedmetal film as an n-side electrode and obviates the necessity to form then-side electrode 18 by exposing the n-type semiconductor layer 12.

Instead of ITO, a translucent multilayer film made of nickel (Ni) andgold (Au) which are stacked in layers may also be used to form thetransparent electrode 16.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 3A through 3C and FIGS. 4A through 4C show the cross-sectionalstructures of the semiconductor light emitting device according to thefirst embodiment in the individual process steps of the fabricationmethod therefor. The drawings show, of a wafer on which a plurality ofsemiconductor light elements can be formed simultaneously, only oneelement formation region.

First, as shown in FIG. 3A, the n-type semiconductor layer 12 made ofn-type GaN, the active layer 13 having a multiple quantum well structuremade of InGaN, the p-type semiconductor layer 14 made of p-typeAl₀₋₀₅Ga_(0.95)N, and the red light emitting layer 15 made of undopedIn_(0.4)Ga_(0.6)N are grown successively by MOCVD (Metal OrganicChemical Vapor Deposition) on the substrate 11 made of sapphire having aprincipal surface of which the plane orientation is, e.g., the (0001)plane. As described above, the active layer 13 is composed of the threequantum well layers each made of In_(0.35)Ga_(0.65)N having a thicknessof 2 nm and the three barrier layers each made of In_(0.02)Ga_(0.98)Nhaving a thickness of 10 nm, which are alternately stacked. However, thestructure of the active layer 13 is not limited thereto provided thatthe emission wavelength is about 470 nM. It is possible to form anunderlie layer made of GaN and a thin-film buffer layer made of GaN orAlN between the substrate 11 and the n-type semiconductor layer 12. Itis also possible to form the active layer 13 such that the compositionof In is nonuniform in the in-plane direction (direction parallel to thesubstrate surface) of the active layer 13. It is also possible to obtainred emission light by using InGaN doped with, e.g., zinc, magnesium, orsilicon, instead of using undoped In_(0.4)Ga_(0.6)N, and thereby formingthe red light emitting layer 15 such that the composition of In is lowerthan 0.4.

Next, as shown in FIG. 3B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 15. Subsequently, the red light emittinglayer 15 and an upper portion of the p-type semiconductor layer 14 areselectively removed by ICP (Inductive Coupled Plasma) etching using,e.g., a chlorine (Cl₂) gas by using the formed metal thin film as amask. At this stage, the portion of the red light emitting layer 15overlying the p-side electrode formation region is also removed.

Next, as shown in FIG. 3C, the metal thin film for the mask is removedand then the transparent electrode 16 which is made of ITO with athickness of about 300 nm and transmits visible light is formed by,e.g., radio-frequency (RF) sputtering over the selectively exposedportions of the p-type semiconductor layer 14 and the red light emittinglayer 15 that has been divided into the plurality of islands eachconfigured as a square when viewed in a plan view. Subsequently, theportion of the transparent electrode 16 overlying the n-side electrodeformation region is removed by wet etching using, e.g., an aqueoushydrogen chloride (HCl) solution. Thereafter, a thermal process isperformed in an oxygen atmosphere at a temperature of, e.g., about 600°C., thereby reducing the contact resistance and the resistivity of thetransparent electrode 16 and improving the transmittance thereof.

Next, as shown in FIG. 4A, the respective portions of the p-typesemiconductor layer 14 and the active layer 13 each overlying the n-sideelectrode formation region 12 a are selectively removed by ICP etching,whereby the n-side electrode formation region 12 a of the n-typesemiconductor layer 12 is exposed.

Next, as shown in FIG. 4B, the n-side electrode 18 as an ohmic electrodecomposed of a multilayer film of titanium (Ti) and gold (Au) is formedby, e.g., sputtering on the n-side electrode formation region 12 a ofthe n-type semiconductor layer 12. Thereafter, sintering may also beperformed appropriately in a nitrogen atmosphere at a temperature of,e.g., about 550° C. to reduce the contact resistance of the n-sideelectrode 18. Subsequently, the p-side electrode 17 made of gold (Au)and serving as a p-side electrode pad is formed selectively by, e.g.,sputtering on the p-side electrode formation region of the transparentelectrode 16. The order in which the n-side electrode 18 and the p-sideelectrode 17 are formed may also be reversed.

Next, as shown in FIG. 4C, after the formation of the p-side electrode17, the resulting structure is divided into light emitting diode chipseach having a 350-μm square size by, e.g., dicing. Subsequently, each ofthe chips resulting from the division is mounted on the specified regionof the package 20 by using, e.g., a silver (Ag) paste. Thereafter, wirebonding is performed with respect to the p-side electrode 17 and then-side electrode 18 and the insulating material 19 containing a YAGfluorescent material is further applied to cover the chip.

Thus, the fabrication method according to the first embodiment allowsone-chip integration of a white light emitting diode in which the redlight emitting layer 15 and the YAG fluorescent material are excited bythe blue light from the blue light emitting diode 10. This enables theintensity of emission light in the red range to be higher than in theconventional white light emitting diode. As a result, it becomespossible to provide a white light emitting diode which outputs whitelight having an excellent color rendering property.

Embodiment 2

A second embodiment of the present invention will be described withreference to the drawings.

FIG. 5 shows a cross-sectional structure of a semiconductor lightemitting device according to the second embodiment. The description ofthe components shown in FIG. 5 which are the same as those shown in FIG.1A will be omitted by retaining the same reference numerals.

As shown in FIG. 5, the blue light emitting diode 10 in which the redlight emitting layer 15 is formed selectively on the p-typesemiconductor layer 14 is mounted by so-called flip-chip mounting suchthat the red light emitting layer 15 is opposed to the mounting surfaceof the package 20.

The p-type semiconductor layer 14 and the red light emitting layer 15are connected electrically to the p-side electrode pad (not shown) ofthe package 20 by the p-side electrode 17 made of a multilayer film ofplatinum (Pt) and gold (Au) and a first bump 22 made of silver. On theother hand, the n-side electrode 18 is connected electrically to then-side electrode pad (not shown) of the package 20 by a second bump 23made of silver (Ag). The blue light emitting diode 10 containing the redlight emitting layer 15 is covered with the insulating material 19containing a YAG fluorescent material.

Thus, in the semiconductor light emitting device according to the secondembodiment, the blue light from the active layer 13 and the red lightemitted from the red light emitting layer 15 through excitation causedby the blue light is released upward through the substrate 11 made ofsapphire. In addition, the blue light outputted from the active layer 13excites a YAG fluorescent material and the excited YAG fluorescentmaterial emits yellow light, as stated previously.

Consequently, even in the structure in which output light is extractedfrom the substrate 11 due to flip-chip mounting, the red light emittinglayer 15 provided in the blue light emitting diode 10 allows theenhancement of an amount of emission light in the red range. As aresult, it becomes possible to provide a white light emitting diodewhich outputs white light having an excellent color rendering property.

In addition, the structure in which output light is extracted from thesubstrate 11 according to the second embodiment obviates the necessityto provide the transparent electrode 16 on the surface of the p-sidesemiconductor layer 14.

Embodiment 3

A third embodiment of the present invention will be described withreference to the drawings.

FIG. 6A shows a cross-sectional structure of a semiconductor lightemitting device according to the third embodiment and FIG. 6B shows aplan structure thereof. The description of the components shown in FIGS.6A and 6B which are the same as those shown in FIGS. 1A and 1B will beomitted by retaining the same reference numerals.

As shown in FIGS. 6A and 6B, the semiconductor light emitting deviceaccording to the third embodiment is composed of: the blue lightemitting diode 10 mounted at a specified position on the package 20 as amounting member; the red light emitting layer 15 grown epitaxially onthe blue light emitting diode 10; and the insulating material 19containing an yttrium aluminum garnet (YAG) fluorescent material.

The third embodiment is different from the first embodiment in that thetransparent electrode 16 made of ITO covers only the upper surface ofthe p-type semiconductor layer 14 without covering the red lightemitting layer 15. Accordingly, an emission spectrum obtained from thesemiconductor light emitting device according to the third embodimentshows a spectrum pattern equal to the pattern shown in FIG. 2.

Thus, one-chip integration of the light emitting device is realized inwhich the red light emitting layer 15 and the YAG fluorescent materialare excited by the blue light from the blue light emitting diode 10.This allows the enhancement of an amount of emission light in the redrange to a value larger than in the conventional white light emittingdiode which provides white light by exciting the YAG fluorescentmaterial with the light emitted from the blue light emitting diode. As aresult, it becomes possible to provide a white light emitting diodehaving an excellent white color rendering property. In addition, sincethe red light emitting layer 15 is not covered with the transparentelectrode 16, the red light is no more absorbed by the transparentelectrode 16 so that red emission light with a higher intensity isobtainable. This allows a white light emitting diode having a moreexcellent color rendering property to be provided.

Preferably, the size of the red light emitting layer 15 having a patternconsisting of discrete islands each configured as a square when viewedin a plan view is determined to allow a current injected from thetransparent electrode 16 into the p-type semiconductor layer 14 to besufficiently diffused in the active layer 13.

In the third embodiment also, an ultraviolet light emitting diode whichoutputs ultraviolet light at a wavelength of, e.g., 340 nm may also beformed in place of the blue light emitting diode 10. In this case, ablue light emitting fluorescent material and a green light emittingfluorescent material are added to the insulating material 19.

It is also possible to separate the substrate 11 made of sapphire fromthe blue light emitting diode 10 and provide a metal film in place ofthe separated substrate. The arrangement allows the provided metal filmto be used as an n-side electrode and obviates the necessity to form then-side electrode 18 by exposing the n-type semiconductor layer 12.

Instead of ITO, a translucent multilayer film composed of nickel (Ni)and gold (Au) which are stacked in layers may also be used to form thetransparent electrode 16. Alternatively, the transparent electrode 16may also be composed of a multilayer film of Ni and ITO.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 7A through 7D and FIGS. 5A through 5C show the cross-sectionalstructures of the semiconductor light emitting device according to thethird embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 7A, the n-type semiconductor layer 12 made ofn-type GaN, the active layer 13 having a multiple quantum well structuremade of InGaN, the p-type semiconductor layer 14 made of p-typeAl_(0.05)Ga_(0.95)N, and the red light emitting layer 15 made of undopedIn_(0.4)Ga_(0.6)N are grown successively by MOCVD on the substrate 11made of sapphire having a principal surface of which the planeorientation is, e.g., the (0001) plane. The active layer 13 is composedof three quantum well layers each made of In_(0.35)Ga_(0.65)N having athickness of 2 nm and three barrier layers each made ofIn_(0.02)Ga_(0.98)N having a thickness of 10 nm, which are alternatelystacked. However, the structure of the active layer 13 is not limitedthereto provided that the emission wavelength is about 470 nm. It ispossible to form an underlie layer made of GaN and a thin-film bufferlayer made of GaN or AlN between the substrate 11 and the n-typesemiconductor layer 12. It is also possible to form the active layer 13such that the composition of In is nonuniform in the in-plane direction(direction parallel to the substrate surface) of the active layer 13. Itis also possible to obtain red emission light by using InGaN doped with,e.g., zinc, magnesium, or silicon, instead of using undopedIn_(0.4)Ga_(0.6)N, and thereby forming the red light emitting layer 15such that the composition of In is lower than 0.4.

Next, as shown in FIG. 7B, a resist film 51 having a pattern consistingof a plurality of discrete and spaced-apart squares each having, e.g.,2-μm to 20-μm sides is formed on the red light emitting layer 15.Subsequently, the red light emitting layer 15 and an upper portion ofthe p-type semiconductor layer 14 are selectively removed by ICP dryetching using, e.g., a chlorine (Cl₂) gas by using the formed resistfilm 51 as a mask. At this stage, the portion of the red light emittinglayer 15 overlying the p-side electrode formation region is alsoremoved.

Next, as shown in FIG. 7C, the transparent electrode 16 which is made ofITO with a thickness of about 300 nm and transmits visible light isformed by, e.g., electron beam vapor deposition on each of theselectively exposed portions of the p-type semiconductor layer 14including the red light emitting layer 15 that has been divided into theplurality of islands each configured as a square when viewed in a planview and the resist film 51 covering the individual islands of the redlight emitting layer 15.

Next, as shown in FIG. 7D, the transparent electrode 16 formedselectively only on the p-type semiconductor layer 14 is obtained by aso-called lift-off process which removes the resist film 51.Subsequently, the portion of the transparent electrode 16 overlying then-side electrode formation region is removed by wet etching using, e.g.,an aqueous hydrogen chloride (HCl) solution. Thereafter, a thermalprocess is performed in an oxygen atmosphere at a temperature of, e.g.,about 600° C., thereby reducing the contact resistance and theresistivity of the transparent electrode 16 and improving thetransmittance thereof.

Next, as shown in FIG. 8A, the respective portions of the p-typesemiconductor layer 14 and the active layer 13 overlying the n-sideelectrode formation region 12 a is selectively removed by ICP etching,whereby the n-side electrode formation region 12 a of the n-typesemiconductor layer 12 is exposed.

Next, as shown in FIG. 5B, the n-side electrode 18 as an ohmic electrodecomposed of a multilayer film of titanium (Ti) and gold (Au) is formedby, e.g., sputtering on the n-side electrode formation region 12 a ofthe n-type semiconductor layer 12. Thereafter, sintering may also beperformed appropriately to reduce the contact resistance of the n-sideelectrode 18 in a nitrogen atmosphere at a temperature of, e.g., about550° C. Subsequently, the p-side electrode 17 made of gold (Au) andserving as a p-side electrode pad is formed selectively by, e.g.,sputtering on the p-side electrode formation region of the transparentelectrode 16. The order in which the n-side electrode 18 and the p-sideelectrode 17 are formed may also be reversed.

Next, as shown in FIG. 8C, the p-side electrode 17 is formed and thendivided into light emitting diode chips each having a 350-μm square sizeby, e.g., dicing. Subsequently, each of the chips resulting from thedivision is mounted on the specified region of the package 20 by using,e.g., a silver (Ag) paste. Thereafter, wire bonding is performed withrespect to the p-side electrode 17 and the n-side electrode 18 and theinsulating material 19 containing a YAG fluorescent material is furtherapplied to cover the chip.

Thus, the fabrication method according to the third embodiment allowsone-chip integration of a white light emitting diode in which the redlight emitting layer 15 and the YAG fluorescent material are excited bythe blue light outputted from the blue light emitting diode 10. Thisenables the intensity of emission light in the red range to be higherthan in the conventional white light emitting diode. As a result, itbecomes possible to provide a white light emitting diode which outputswhite light having an excellent color rendering property.

In addition, since the red light emitting layer 15 is not covered withthe transparent electrode 16, there is no more absorption of the redlight by the transparent electrode 16 so that red emission light with ahigher intensity is obtainable. This provides white light having a moreexcellent color rendering property.

Embodiment 4

A fourth embodiment of the present invention will be described withreference to the drawings.

FIG. 9A shows a cross-sectional structure of a semiconductor lightemitting device according to the fourth embodiment and FIG. 9B shows aplan structure thereof. The description of the components shown in FIGS.9A and 9B which are the same as those shown in FIGS. 1A and 1B will beomitted by retaining the same reference numerals.

As shown in FIGS. 9A and 9B, the semiconductor light emitting deviceaccording to the fourth embodiment is composed of: the blue lightemitting diode 10; a green light emitting layer 24 grown epitaxially onthe blue light emitting diode 10; and the red light emitting layer 15grown epitaxially on the green light emitting layer 24.

The fourth embodiment is different from the first embodiment in that itprovides the green light emitting layer 24 which is made ofIn_(0.2)Ga_(0.5)N and excited by blue light at a wavelength of 470 nmemitted from the blue light emitting diode 10 to emit green light at awavelength of 555 nm and thereby obviates the necessity for theinsulating material 19 covering the blue light emitting diode 10 andcontaining a YAG fluorescent material which emits yellow light.

Green light and red light can be obtained from the green light emittinglayer 24 and the red light emitting layer 15 by doping each of the redlight emitting layer 15 and the green light emitting layer 24 with,e.g., zinc (Zn), magnesium (Mg), or silicon (Si) and thereby reducingthe composition of In in each of the light emitting layers. By thusreducing the composition of In, a lattice mismatch with an underlielayer made of GaN normally used can be suppressed and crystal defects inthe green light emitting layer 24 and the red light emitting layer 15can be reduced so that high-brightness light emission is enabled.

White light having the spectrum shown in FIG. 10 can be obtained byinjecting a current of, e.g., 20 mA into the blue light emitting diode10 and thereby causing the emission of blue light at a wavelength of 470nm. In FIG. 10, the emission spectrum is composed of the transmittedcomponent 10A of the blue light at a wavelength of 470 nm, greenemission light 24A with a peak wavelength of 555 nm from the green lightemitting layer 24, and the red light 15A at a wavelength of 650 nm fromthe red light emitting layer 15. The blue light 10A, the green light24A, and the red light 15A are mixed to provide white light.

Thus, the fourth embodiment allows one-chip integration of a lightemitting diode in which the green light emitting layer 24 and the redlight emitting layer 15 which receive blue light outputted from the bluelight emitting diode 10 and generate green light and red light throughexcitation caused by the received blue light are provided on the bluelight emitting diode 10. Accordingly, the intensity of emission light inthe red range is higher than in an emission spectrum obtained from theconventional white light emitting diode which provides white light byexciting the YAG fluorescent material with blue light from the bluelight emitting diode. This allows a white light emitting diode whichoutputs white light having an excellent color rendering property to beprovided.

Although white light has been obtained in the fourth embodiment throughthe reception of the output light from the blue light emitting diode 10,the excitation of the green light emitting layer 24 and the red lightemitting layer 15, and the emission of green light and red lighttherefrom, an ultraviolet light emitting diode which outputs ultravioletlight at a wavelength of, e.g., 340 nm may also be formed in place ofthe blue light emitting diode 10. In this case, the ultraviolet lightemitting diode is covered with an insulating material containing a bluelight emitting fluorescent material.

It is also possible to separate the substrate 11 made of sapphire fromthe blue light emitting diode 10 and provide a metal film in place ofthe separated substrate. The arrangement allows the provided metal filmto be used as an n-side electrode and obviates the necessity to form then-side electrode 18 by exposing the n-type semiconductor layer 12.

Instead of ITO, a translucent multilayer film composed of nickel (Ni)and gold (Au) which are stacked in layers may also be used to form thetransparent electrode 16. Alternatively, the transparent electrode 16may also be composed of a multilayer film of Ni and ITO.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 11A through 11C and FIGS. 12A and 12B show the cross-sectionalstructures of the semiconductor light emitting device according to thefourth embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 11A, the n-type semiconductor layer 12 made ofn-type GaN, the active layer 13 having a multiple quantum well structuremade of InGaN, the p-type semiconductor layer 14 made of p-typeAl_(0.05)Ga_(0.95)N, the green light emitting layer 24 made of undopedIn_(0.02)Ga_(0.8)N, and the red light emitting layer 15 made of undopedIn_(0.4)Ga_(0.6)N are grown successively by MOCVD on the substrate 11made of sapphire having a principal surface of which the planeorientation is, e.g., the (0001) plane. The active layer 13 is composedof three quantum well layers each made of In_(0.35)Ga_(0.65)N having athickness of 2 nm and three barrier layers each made ofIn_(0.02)Ga_(0.98)N having a thickness of 10 nm, which are alternatelystacked. However, the structure of the active layer 13 is not limitedthereto provided that the emission wavelength is about 470 nm. Theforbidden band width of the green light emitting layer 24 is 2.3 eV andemits green light at 555 nm. The forbidden band width of the red lightemitting layer 15 is 1.9 eV and emits red light at 650 nm. It ispossible to form an underlie layer made of GaN and a thin-film bufferlayer made of GaN or AlN between the substrate 11 and the n-typesemiconductor layer 12. It is also possible to obtain green emissionlight and red emission light by using InGaN doped with, e.g., zinc,magnesium, or silicon, instead of using undoped In_(0.2)Ga_(0.8)N andIn_(0.4)Ga_(0.6)N, and thereby forming the green light emitting layer 24and the red light emitting layer 15 such that the composition of In islower than 0.2 and 0.4, respectively.

Next, as shown in FIG. 11B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 15. Subsequently, the red light emittinglayer 15, the green light emitting layer 24, and an upper portion of thep-type semiconductor layer 14 are selectively removed by ICP etchingusing, e.g., a chlorine (Cl₂) gas by using the formed metal thin film asa mask. At this stage, the respective portions of the red light emittinglayer 15 and the green light emitting layer 24 overlying the p-sideelectrode formation region are also removed.

Next, as shown in FIG. 11C, the metal thin film is removed and then thetransparent electrode 16 which is made of ITO with a thickness of about300 mm and transmits visible light is formed by, e.g., RF sputteringover the selectively exposed portions of the p-type semiconductor layer14 and the red light emitting layer 15 that has been divided into theplurality of islands each configured as a square when viewed in a planview. Subsequently, the portion of the transparent electrode 16overlying the n-side electrode formation region is removed by wetetching using, e.g., an aqueous hydrogen chloride (HCl) solution.Thereafter, a thermal process is performed in an oxygen atmosphere at atemperature of, e.g., about 600° C., thereby reducing the contactresistance and the resistivity of the transparent electrode 16 andimproving the transmittance thereof.

Next, as shown in FIG. 12A, the respective portions of the p-typesemiconductor layer 14 and the active layer 13 overlying the n-sideelectrode formation region 12 a are selectively removed by ICP etching,whereby the n-side electrode formation region 12 a of the n-typesemiconductor layer 12 is exposed.

Next, as shown in FIG. 12B, the n-side electrode 18 as an ohmicelectrode composed of a multilayer film of titanium (Ti) and gold (Au)is formed by, e.g., sputtering on the exposed n-side electrode formationregion 12 a of the n-type semiconductor layer 12. Thereafter, sinteringmay also be performed appropriately to reduce the contact resistance ofthe n-side electrode 18 in a nitrogen atmosphere at a temperature of,e.g., about 550° C. Subsequently, the p-side electrode 17 made of gold(Au) and serving as a p-side electrode pad is formed selectively by,e.g., sputtering on the p-side electrode formation region of thetransparent electrode 16. The order in which the n-side electrode 18 andthe p-side electrode 17 are formed may also be reversed. Thereafter, theresulting structure is divided into light emitting diode chips eachhaving a 350-μm square size by, e.g., dicing. Subsequently, each of thechips resulting from the division is mounted on the specified region ofa package (not shown) by using, e.g., a silver (Ag) paste. Thereafter,wire bonding is performed with respect to the p-side electrode 17 andthe n-side electrode 18.

Thus, the fabrication method according to the fourth embodiment allowsone-chip integration of a white light emitting diode in which the greenlight emitting layer 24 and the red light emitting layer 15 are excitedby the blue light from the blue light emitting diode 10. This enablesthe intensity of emission light in the red range to be higher than inthe conventional white light emitting diode. As a result, it becomespossible to provide a white light emitting diode which outputs whitelight having an excellent color rendering property.

In addition, the step of applying the insulating material containing theYAG fluorescent material can be eliminated so that the fabricationprocess is simplified.

Embodiment 5

A fifth embodiment of the present invention will be described withreference to the drawings.

FIG. 13A shows a cross-sectional structure of a semiconductor lightemitting device according to the fifth embodiment and FIG. 13B shows aplan structure thereof.

As shown in FIGS. 13A and 13B, the semiconductor light emitting deviceaccording to the fifth embodiment is composed of: an ultraviolet lightemitting diode 30; a blue light emitting layer 25; the green lightemitting layer 24; and the red light emitting layer 15, each of whichhas been grown epitaxially on the ultraviolet light emitting diode 30.

The ultraviolet light emitting diode 30 is composed of: an n-typesemiconductor layer 32 made of, e.g., n-type Al_(0.1)Ga_(0.9)N; anactive layer 33 having a multiple quantum well structure made of InGaNand AlGaN; and a p-type semiconductor layer 34 made of p-typeAl_(0.15)Ga_(0.85)N. The active layer 33 is composed of five well layerseach made of In_(0.02)Ga_(0.98)N having a thickness of 1.5 nm and fivebarrier layers each made of Al_(0.15)Ga_(0.05)N having a thickness of 10nm which are alternately stacked, thereby emitting ultraviolet light at340 nm.

The n-side electrode 18 composed of a multilayer structure of titanium(Ti) and gold (Au) has been formed on the entire surface of the n-typesemiconductor layer 32 opposite to the active layer 33. A plating layer31 made of gold (Au) with a thickness of 10 μm or more, e.g., about 50μm has been formed on the entire surface of the n-side electrode 18opposite to the n-type semiconductor layer 32 to substantially functionas the n-side electrode. Preferably, a material having a reflectivity of60% or more with respect to ultraviolet light, blue light, green light,and red light is used herein for the n-side electrode 18. For example, asingle layer film made of, e.g., gold (Au), platinum (Pt), copper (Cu),silver (Ag), or rhodium (Rh) or a multilayer film containing at leasttwo of the foregoing elements can be used. For the gold plating layer31, copper (Cu) or silver (Ag) can be used instead of gold (Au).

The blue light emitting layer 25 is made of, e.g., undopedIn_(0.15)Ga_(0.85)N with a forbidden band width of 2.6 eV and formed onthe p-type semiconductor layer 34 to have a configuration consisting ofa plurality of discrete and spaced-apart islands. The green lightemitting layer 24 is made of, e.g., undoped In_(0.2)Ga_(0.8)N with aforbidden band width of 2.2 eV and formed on the blue light emittinglayer 25 to have the same plan configuration as the blue light emittinglayer 25. The red light emitting layer 15 is made of, e.g., undopedIn_(0.4)Ga_(0.6)N with a forbidden band width of 1.9 eV and formed onthe green light emitting layer 24 to have the same plan configuration asthe green light emitting layer 24. The blue light emitting layer 25, thegreen light emitting layer 24, and the red light emitting layer 15 areexcited by the ultraviolet light outputted from the ultraviolet lightemitting diode 30 to emit blue light at a wavelength of 470 nm, greenlight at a wavelength of 555 nm, and red light at a wavelength of 650nm. Blue light emission, green light emission, and red light emissioncan be obtained from the blue light emitting layer 25, the green lightemitting layer 24, and the red light emitting layer 15 by doping each ofthe blue light emitting layer 25, the green light emitting layer 24, andthe red light emitting layer 15 with, e.g., zinc (Zn), magnesium (Mg),or silicon (Si) and thereby reducing the composition of In in each ofthe light emitting layers. By thus reducing the composition of In, alattice mismatch with an underlie layer made of GaN normally used can besuppressed and crystal defects in the blue light emitting layer 25, inthe green light emitting layer 24, and in the red light emitting layer15 can be reduced so that high-brightness light emission is enabled.

On the entire surface of the p-type semiconductor layer 34 including theblue light emitting layer 25, the green light emitting layer 24, and thered light emitting layer 15, the transparent electrode made of ITO hasbeen formed. Further, the p-side electrode 17 made of gold (Au) has beenformed on a region of the p-type semiconductor layer 34 with thetransparent electrode 16 being interposed therebetween.

A current is injected into the ultraviolet light emitting diode 30 viathe p-side electrode 17, the transparent electrode 16, and the p-typesemiconductor layer 34. The ultraviolet light emitting diode 30 iscapable of operating at a relatively low voltage of, e.g., about 3 V.Accordingly, white light having the spectrum shown in FIG. 14 can beobtained by injecting a current of, e.g., 20 mA into the ultravioletlight emitting diode 30 and thereby causing the emission of ultravioletlight at a wavelength of 340 μm. In FIG. 14, the emission spectrum iscomposed of the transmitted component 30A of the ultraviolet light at awavelength of 340 nm which is low in intensity, blue light 25A with apeak wavelength of 470 nm from the blue light emitting layer 25, greenlight 24A with a peak wavelength of 555 nm from the green light emittinglayer 24, and the red light 15A at a wavelength of 650 nm from the redlight emitting layer 15. The blue light 25A, the green light 24A, andthe red light 15A are mixed to provide white light.

The fifth embodiment is characterized in that the n-side electrode 18 isformed over the entire surface (back surface) of the n-typesemiconductor layer 32 opposite to the active layer 33 by removing thesubstrate made of sapphire for epitaxial growth and the gold platinglayer 31 is further provided. The arrangement remarkably improves theheat dissipation property of the ultraviolet light emitting diode 30 andallows a higher-output white light emitting diode to be provided. Inaddition, since the n-side electrode 18 and the p-side electrode 17 aredisposed in opposing relation with the active layer 33 interposedtherebetween, a series resistance between the n-side electrode 18 andthe p-side electrode 17 can be reduced advantageously. Since theinsulating substrate made of sapphire or the like has been removed, itis unnecessary to provide the n-side electrode 18 on the upper portionof the n-type semiconductor layer 32. This achieves a reduction in chipsize and allows the elimination of the step of etching away the n-sidesemiconductor layer 32 from the side of the p-type semiconductor layer34.

Thus, the fifth embodiment allows one-chip integration of a lightemitting diode in which the blue light emitting layer 25, the greenlight emitting layer 24, and the red light emitting layer 15 whichreceive ultraviolet light outputted from the ultraviolet light emittingdiode 30 and generate blue light, green light, and red light throughexcitation caused by the received ultraviolet light are provided on theultraviolet light emitting diode 30. Accordingly, the intensity ofemission light in the red range is higher than in an emission spectrumobtained from the conventional white light emitting diode which provideswhite light by exciting the YAG fluorescent material with blue lightfrom the blue light emitting diode. This allows a white light emittingdiode which outputs white light having an excellent color renderingproperty to be implemented.

In addition, the substrate made of sapphire which is not excellent inheat dissipation property is removed and the gold plating layer 31 whichis excellent in heat dissipation property is provided in place thereof.As a result, a white light emitting diode featuring a higher output andan excellent color rending property can be provided.

In the case where the ultraviolet light emitting diode 30 is replacedwith the blue light emitting diode 10 used in the fourth embodiment, theblue light emitting layer 25 need not be provided.

Instead of ITO, a translucent multilayer film composed of nickel (Ni)and gold (Au) which are stacked in layers may also be used to form thetransparent electrode 16. Alternatively, the transparent electrode 16may also be composed of a multilayer film of Ni and ITO.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 15A through 15E and FIGS. 16A through 16C show the cross-sectionalstructures of the semiconductor light emitting device according to thefifth embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 15A, the n-type semiconductor layer 32 made ofn-type Al_(0.1)Ga_(0.9)N, the active layer 33 having a multiple quantumwell structure made of InGaN and AlGaN, the p-type semiconductor layer34 made of p-type Al_(0.15)Ga_(0.85)N, the blue light emitting layer 25made of undoped In_(0.15)Ga_(0.85)N, the green light emitting layer 24made of undoped In_(0.2)Ga_(0.8)N, and the red light emitting layer 15made of undoped In_(0.4)Ga_(0.6)N are grown successively by MOCVD on thesubstrate 11 made of sapphire having a principal surface of which theplane orientation is, e.g., the (0001) plane. The active layer 33 iscomposed of five quantum well layers each made of In_(0.2)Ga_(0.98)Nhaving a thickness of 1.5 nm and five barrier layers each made ofAl_(0.15)Ga_(0.85)N having a thickness of 10 nm, which are alternatelystacked. However, the structure of the active layer 33 is not limitedthereto provided that the emission wavelength is about 470-nm. Theforbidden band width of the blue light emitting layer 25 is 2.6 eV andemits blue light at 340 nm. The forbidden band width of the green lightemitting layer 24 is 2.3 eV and emits green light at 555 nm. Theforbidden band width of the red light emitting layer 15 is 1.9 eV andemits red light at 650 nm. It is possible to form an underlie layer madeof GaN and a thin-film buffer layer made of GaN or AlN between thesubstrate 11 and the n-type semiconductor layer 32. It is also possibleto obtain blue light emission, green light emission, and red lightemission by using InGaN doped with, e.g., zinc, magnesium, or silicon inwhich the composition of In is lower than 0.2, instead of using undopedIn_(0.15)Ga_(0.85)N, In_(0.2)Ga_(0.8)N, and In_(0.4)Ga_(0.6)N, andthereby forming the blue light emitting layer 25, the green lightemitting layer 24, and the red light emitting layer 15 such that thecomposition of In is lower than 0.4.

Next, as shown in FIG. 15B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 15. Subsequently, the red light emittinglayer 15, the green light emitting layer 24, the blue light emittinglayer 25, and an upper portion of the p-type semiconductor layer 14 areremoved selectively by ICP etching using, e.g., a chlorine (Cl₂) gas byusing the formed metal thin film as a mask. At this stage, therespective portions of the red light emitting layer 15, the green lightemitting layer 24, and the blue light emitting layer 25 overlying thep-side electrode formation region are also removed.

Next, as shown in FIG. 15C, the metal thin film is removed and then thetransparent electrode 16 which is made of ITO with a thickness of about300 nm and transmits visible light is formed by, e.g., RF sputteringover the selectively exposed portions of the p-type semiconductor layer14 and the red light emitting layer 15 that has been divided into theplurality of islands each configured as a square when viewed in a planview. Subsequently, a thermal process is performed in an oxygenatmosphere at a temperature of, e.g., about 600° C., thereby reducingthe contact resistance and the resistivity of the transparent electrode16 and improving the transmittance thereof.

Next, as shown in FIG. 15D, the p-side electrode 17 made of gold (Au)and serving as a p-side electrode pad is formed selectively by, e.g.,sputtering on the p-side electrode formation region of the transparentelectrode 16.

Next, as shown in FIG. 15E, after the formation of the p-side electrode17, a holding substrate 52 made of silicon and the transparent electrode16 including the p-side electrode are bonded to each other by using,e.g., an epoxy-based adhesive agent 53. The material of the holdingsubstrate 52 is not limited to silicon. A polymer film may also be usedfor the holding substrate 52.

Next, as shown in FIG. 16A, a high-output and short-wavelength pulselaser beam which is not absorbed by the substrate 11 but is absorbed bythe n-type semiconductor layer 32, such as the third-harmonic beam of aYAG laser at a wavelength of, e.g., 355 nm or a KrF excimer laser beamat a wavelength of 248 nm, is applied in a scanning manner to thesurface of the substrate 11 opposite to the holding substrate 52 for theirradiation thereof. At this time, the applied laser beam is absorbed bythe portion of the n-type semiconductor layer 32 made ofAl_(0.1)Ga_(0.9)N which is adjacent to the interface between itself andthe substrate 11. As a result, the portion of the n-type semiconductorlayer 32 which is adjacent to the interface with the substrate 11 isheated and, if the temperature becomes, e.g., 900° C. or higher throughthe absorption of the laser beam, the portion of the n-typesemiconductor layer 32 adjacent to the interface with the substrate 11is decomposed into a metal gallium (Ga) gas, a metal aluminum (Al) gas,and a nitrogen (N₂) gas, so that a decomposition layer is formed. In thecase of using a YAG laser as the laser beam for forming thedecomposition layer, a semiconductor thin film made of gallium nitride(GaN) is inserted between the substrate 11 and the n-type semiconductorlayer 32 to accelerate the absorption of the YAG laser beam and thedecomposition layer is formed by irradiating the inserted semiconductorthin film with the YAG laser beam.

Then, the substrate 11 formed with the decomposition layer is separatedfrom the n-type semiconductor layer 32 by heating the substrate 11 to atemperature not less than 29° C., which is a melting point of gallium,or by immersing the substrate 11 in an aqueous hydrogen chloride (HCl)solution and thereby melting or removing metal gallium contained in thedecomposition layer. Thereafter, the n-side electrode 18 composed of amultilayer film of titanium (Ti) and gold (Au) is formed by, e.g.,electron beam vapor deposition on the exposed surface from which thesubstrate 11 has been separated and removed. Subsequently, the goldplating layer 31 with a thickness of about 50 μm is formed byelectrolytic plating using the gold (Au) layer of the n-side electrode18 as an underlie, whereby the structure shown in FIG. 16B is obtained.

Next, as shown in FIG. 16C, the adhesive agent 53 is removed by using,e.g., acetone so that the holding substrate 52 is removed. Then, theresulting structure is divided into light emitting diode chips eachhaving a 350-μm square size by, e.g., dicing. Subsequently, each of thechips resulting from the division is mounted on the specified region ofa package (not shown) by using, e.g., a silver (Ag) paste. Thereafter,wire bonding is performed with respect to the p-side electrode 17,whereby the white light emitting diode is obtained.

Thus, the fabrication method according to the fifth embodiment allowsone-chip integration of a white light emitting diode in which the bluelight emitting layer 25, the green light emitting layer 24, and the redlight emitting layer 15 are excited by the ultraviolet light outputtedfrom the ultraviolet light emitting diode 30. This enables the intensityof emission light in the red range to be higher than in the conventionalwhite light emitting diode. As a result, it becomes possible to providea white light emitting diode which outputs white light having anexcellent color rendering property.

In addition, the step of applying the insulating material containing theYAG fluorescent material can be eliminated so that the fabricationprocess is simplified. Moreover, the sapphire substrate 11 which is notexcellent in heat dissipation property is removed and the gold platinglayer 31 which is excellent in heat dissipation property is provided inplace thereof so that a higher output is produced.

Embodiment 6

A sixth embodiment of the present invention will be described withreference to the drawings.

FIG. 17A shows a cross-sectional structure of a semiconductor lightemitting device according to the sixth embodiment and FIG. 17B shows aplan structure thereof. The description of the components shown in FIGS.17A and 17B which are the same as those shown in FIGS. 13A and 13B willbe omitted by retaining the same reference numerals.

The semiconductor light emitting device according to the sixthembodiment is the same as the semiconductor light emitting deviceaccording to the fifth embodiment in that it is composed of theultraviolet light emitting diode 30, the blue light emitting layer 25,the green light emitting layer 24, and the red light emitting layer 15each formed thereon. The semiconductor light emitting device accordingto the sixth is different from the semiconductor light emitting deviceaccording to the fifth embodiment in that the transparent electrode 16is formed on the p-type semiconductor layer 34 to have a plurality ofopenings 16 a each configured as a square when viewed in a plan view andthe blue light emitting layer 25 is grown epitaxially by using thetransparent electrode 16 having the plurality of openings 16 a as a maskfor selective growth. Accordingly, an emission spectrum obtained fromthe semiconductor light emitting device according to the sixthembodiment shows a spectrum pattern equal to the pattern shown in FIG.14.

The injection of a current into the ultraviolet light emitting diode 30is performed via the transparent electrode 16 that has been patterned tobe used as the mask for selective growth and the p-type semiconductorlayer 34.

Since an area occupied by the blue light emitting layer 25, the greenlight emitting layer 24, and the red light emitting layer 15 can beincreased in the sixth embodiment, the intensity of white light can beincreased.

In addition, the portion grown laterally over the transparent electrode16 used as the mask for selective growth, i.e., in a direction parallelto the upper surface of the transparent electrode 16 has an improvedcrystal property so that a crystal dislocation density is low. As aresult, high-brightness visible light can be emitted through excitationcaused by the ultraviolet light outputted from the ultraviolet lightemitting diode 30.

In addition, the n-side electrode 18 is formed over the entire surface(back surface) of the n-type semiconductor layer 32 opposite to theactive layer 33 by removing the substrate made of sapphire for epitaxialgrowth and the gold plating layer 31 is further provided. Thearrangement remarkably improves the heat dissipation property of theultraviolet light emitting diode 30 and allows a higher-output whitelight emitting diode to be provided. In addition, since the n-sideelectrode 18 and the p-side electrode 17 are disposed in opposingrelation with the active layer 33 interposed therebetween, a seriesresistance between the n-side electrode 18 and the p-side electrode 17can be reduced advantageously. Since the insulating substrate made ofsapphire or the like has been removed, it is unnecessary to provide then-side electrode 18 on the upper portion of the n-type semiconductorlayer 32. This achieves a reduction in chip size and allows theelimination of the step of etching away the n-side semiconductor layer32 from the side of the p-type semiconductor layer 34.

Thus, the sixth embodiment allows one-chip integration of a white lightemitting diode in which the blue light emitting layer 25, the greenlight emitting layer 24, and the red light emitting layer 15 whichreceive ultraviolet light outputted from the ultraviolet light emittingdiode 30 and generate blue light, green light, and red light throughexcitation caused by the received light are provided on the ultravioletlight emitting diode 30. Accordingly, the intensity of emission light inthe red range is higher than in an emission spectrum obtained from theconventional white light emitting diode which provides white light byexciting the YAG fluorescent material with blue light from the bluelight emitting diode. This allows a white light emitting diode whichoutputs white light having an excellent color rendering property to beprovided.

In addition, the substrate made of sapphire which is not excellent inheat dissipation property is removed and the gold plating layer 31 whichis excellent in heat dissipation property is provided in place thereof.As a result, a white light emitting diode featuring a higher output andan excellent color rending property can be provided.

In the case where the ultraviolet light emitting diode 30 is replacedwith the blue light emitting diode 10 used in the fourth embodiment, theblue light emitting layer 25 need not be provided.

Instead of ITO, a translucent multilayer film composed of nickel (Ni)and gold (Au) which are stacked in layers may also be used to form thetransparent electrode 16. Alternatively, the transparent electrode 16may also be composed of a multilayer film of Ni and ITO.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 18A through 18D and FIGS. 19A through 19C show the cross-sectionalstructures of the semiconductor light emitting device according to thesixth embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 18A, the n-type semiconductor layer 32 made ofn-type Al_(0.1)Ga_(0.9)N, the active layer 33 having a multiple quantumwell structure made of InGaN and AlGaN and the p-type semiconductorlayer 34 made of p-type Al_(0.15)Ga_(0.85) are grown successively byMOCVD on the substrate 11 made of sapphire having a principal surface ofwhich the plane orientation is, e.g., the (0001) plane. The active layer33 is composed of five quantum well layers each made ofIn_(0.02)Ga_(0.98)N having a thickness of 1.5 nm and five barrier layerseach made of Al_(0.15)Ga_(0.85)N having a thickness of 10 nm, which arealternately stacked. However, the structure of the active layer 33 isnot limited thereto provided that the emission wavelength is about 340nm. Subsequently, the transparent electrode 16 made of ITO with athickness of about 300 nm is formed by, e.g., RF sputtering on thep-type semiconductor layer 34 and then the plurality of openings 16 aeach configured as a square with 2-μm to 20-μm sides when viewed in aplan view are formed in the transparent electrode 16 by using, e.g., anaqueous hydrofluoric acid (HF) solution. Thereafter, a thermal processis performed in an oxygen atmosphere at a temperature of, e.g., about600° C., thereby reducing the contact resistance and the resistivity ofthe transparent electrode 16 and improving the transmittance thereof.

Next, as shown in FIG. 18B, the blue light emitting layer 25 made ofundoped In_(0.15)Ga_(0.85)N, the green light emitting layer 24 made ofundoped In_(0.2)Ga_(0.8)N, and the red light emitting layer 15 made ofundoped In_(0.4)Ga_(0.6)N are grown successively by MOCVD on the p-typesemiconductor layer 34 by using the transparent electrode 16 formed withthe plurality of openings 16 a as a mask. At this time, the size of eachof the openings 16 a provided in the transparent electrode 16 ispreferably minimized within a range which allows planar growth of theblue light emitting layer 25 because, e.g., a crystal defect is likelyto occur in the portion of the blue light emitting layer 25 overlyingthe opening 16 a. The forbidden band width of the blue light emittinglayer 25 is 2.6 eV herein and emits blue light at 470 nm. The forbiddenband width of the green light emitting layer 24 is 2.3 eV and emitsgreen light at 555 nm. The forbidden band width of the red lightemitting layer 15 is 1.9 eV and emits red light at 650 nm. It ispossible to form an underlie layer made of GaN and a thin-film bufferlayer made of GaN or AlN between the substrate 11 and the n-typesemiconductor layer 32. It is also possible to obtain blue lightemission, green light emission, and red light emission by using InGaNdoped with, e.g., zinc, magnesium, or silicon, instead of using undopedIn_(0.15)Ga_(0.85)N, In_(0.2)Ga_(0.8)N, and In_(0.4)Ga_(0.6)N, andthereby forming the blue light emitting layer 25, the green lightemitting layer 24, and the red light emitting layer 15 such that thecomposition of In is lower than 0.2 and 0.4, respectively.

Next, as shown in FIG. 18C, the p-side electrode 17 made of gold (Au)and serving as a p-side electrode pad is formed selectively by, e.g.,sputtering on the p-side electrode formation region of the transparentelectrode 16.

Next, as shown in FIG. 18D, after the formation of the p-side electrode17, a holding substrate 52 made of silicon, the transparent electrode 16including the p-side electrode, and the red light emitting layer 15 arebonded to each other by using, e.g., an epoxy-based adhesive agent 53.The material of the holding substrate 52 is not limited to silicon. Apolymer film may also be used for the holding substrate 52.

Next, as shown in FIG. 19A, a high-output and short-wavelength pulselaser beam, such as the third-harmonic beam of a YAG laser at awavelength of 355 nm or a KrF excimer laser beam at a wavelength of 248nm, is applied in a scanning manner to the surface of the substrate 11opposite to the holding substrate 52 for the irradiation thereof. Atthis time, the applied laser beam is absorbed by the portion of then-type semiconductor layer 32 made of Al_(0.1)Ga_(0.9)N which isadjacent to the interface between itself and the substrate 11. As aresult, the portion of the n-type semiconductor layer 32 which isadjacent to the interface with the substrate 11 is heated and, if thetemperature becomes 900° C. or higher through the absorption of thelaser beam, the portion of the n-type semiconductor layer 32 adjacent tothe interface with the substrate 11 is decomposed into a metal gallium(Ga) gas, a metal aluminum (Al) gas, and a nitrogen (N₂) gas, so that adecomposition layer is formed. In the case of using a YAG laser as thelaser beam for forming the decomposition layer, a semiconductor thinfilm made of gallium nitride (GaN) is inserted between the substrate 11and the n-type semiconductor layer 32 to accelerate the absorption ofthe YAG laser beam and the decomposition layer is formed by irradiatingthe inserted semiconductor thin film with the YAG laser beam.

Then, the substrate 11 formed with the decomposition layer is separatedfrom the n-type semiconductor layer 32 by heating the substrate 11 to atemperature not less than 29° C., which is a melting point of gallium,or by immersing the substrate 11 in an aqueous hydrogen chloride (HCl)solution and thereby melting or removing metal gallium contained in thedecomposition layer. Thereafter, the n-side electrode 18 composed of amultilayer film of titanium (Ti) and gold (Au) is formed by, e.g.,electron beam vapor deposition on the exposed surface from which thesubstrate 11 has been separated and removed. Subsequently, the goldplating layer 31 with a thickness of about 50 μm is formed byelectrolytic plating using the gold (Au) layer of the n-side electrode18 as an underlie, whereby the structure shown in FIG. 19B is obtained.

Next, as shown in FIG. 19C, the adhesive agent 53 is removed by using,e.g., acetone so that the holding substrate 52 is removed. Then, theresulting structure is divided into light emitting diode chips eachhaving a 350-μm square size by, e.g., dicing. Subsequently, each of thechips resulting from the division is mounted on the specified region ofa package (not shown) by using, e.g., a silver (Ag) paste. Thereafter,wire bonding is performed with respect to the p-side electrode 17,whereby the white light emitting diode is obtained.

Thus, the fabrication method according to the sixth embodiment allowsone-chip integration of a white light emitting diode in which the bluelight emitting layer 25, the green light emitting layer 24, and the redlight emitting layer 15 are excited by the ultraviolet light outputtedfrom the ultraviolet light emitting diode 30. This enables the intensityof emission light in the red range to be higher than in the conventionalwhite light emitting diode. As a result, it becomes possible to providea white light emitting diode which outputs white light having anexcellent color rendering property.

In addition, since the transparent electrode 16 does not cover the bluelight emitting layer 25, the green light emitting layer 24, and the redlight emitting layer 15, the brightness of each of the blue emissionlight, the green emission light, and the red emission light is enhanced.

Moreover, since the step of applying the insulating material containingthe YAG fluorescent material can be eliminated, the fabrication processis simplified. Furthermore, the sapphire substrate 11 which is notexcellent in heat dissipation property is removed and the gold platinglayer 31 which is excellent in heat dissipation property is provided inplace thereof so that a higher output is produced.

Although each of the fifth and sixth embodiments has described theseparation method which applies the high-output short-wavelength pulselaser beam for the separation of the substrate 11 made of sapphire, theseparation of the substrate 11 is not limited to the method using laserirradiation. It is also possible to, e.g., use the substrate 11 made ofsilicon (Si) or gallium arsenide (GaAs) in place of the substrate 11made of sapphire and separate and remove the substrate 11 by wet etchingusing an acid.

Embodiment 7

A seventh embodiment of the present invention will be described withreference to the drawings.

FIG. 20 shows a cross-sectional structure of a semiconductor lightemitting device according to the seventh embodiment and FIG. 21 shows aplan structure thereof when viewed from the side with the electrode. Thedescription of the components shown in FIGS. 21 and 22 which are thesame as those shown in FIGS. 1A and 1B will be omitted by retaining thesame reference numerals.

The semiconductor light emitting device according to the seventhembodiment is the same as the semiconductor light emitting deviceaccording to the first embodiment in that it is composed of the bluelight emitting diode 10, the red light emitting layer 15 grownepitaxially on the blue light emitting diode 10, and the insulatingmaterial 19 containing an yttrium aluminum garnet (YAG) fluorescentmaterial. In FIG. 21 also, the depiction of the insulating material 19is omitted.

The semiconductor light emitting device according to the seventhembodiment is different from the semiconductor light emitting deviceaccording to the first embodiment in that a red light emitting Eu dopedlayer 150 made of undoped Al_(0.5)Ga_(0.5)N is provided in place of thered light emitting layer 15 made of undoped InGaN.

The red light emitting Eu doped layer 150 has been formed by implantingeuropium (Eu) ions into the epitaxially grown AlGaN layer under suchimplant conditions that, e.g., an acceleration voltage is about 200 keVand a dose is about 1×10¹⁵ cm⁻². Under these implant conditions, asshown in FIG. 22, the red light emitting Eu doped layer 150 shows arelatively shallow Eu concentration profile having a peak of 1×10²⁰ cm⁻³at a depth of about 75 nm from the surface of the AlGaN layer. It willeasily be appreciated that the Eu concentration profile can be changedby adjusting the acceleration voltage and the dose.

When the red light emitting Eu doped layer 150 receives visible light orultraviolet light, inner-shell electrons in Eu atoms used for doping areexcited and release red light at a wavelength of 622 nm on returningfrom the excited state to a ground state. By increasing the dose ofimplanted Eu ions, the emission intensity of the red excitation lightcan be enhanced.

The semiconductor composing the red light emitting Eu doped layer 150 isnot limited to AlGaN. Ga may also be used instead. It is also possibleto use a semiconductor layer having a multiple quantum well layercomposed of a pair of In_(0.02)Ga_(0.98)N and Al_(0.4)Ga_(0.6)N.

The doping of the semiconductor layer with Eu is not limited to a methodwhich performs ion implantation after epitaxial growth. Doping with Eumay also be performed during epitaxial growth.

White light having the spectrum shown in FIG. 23 can be obtained byinjecting a current of, e.g., 20 mA into the blue light emitting diode10 and thereby causing the emission of blue light at a wavelength of 470nm. In FIG. 23, the emission spectrum is composed of the transmittedcomponent 10A of the blue light at a wavelength of 470 nm, yellowemission light 19A with a peak wavelength of 550 nm from the YAGfluorescent material, and red light 150A at a wavelength of 622 nm fromthe red light emitting Eu doped layer 150. The blue light 10A, theyellow light 19A, and the red light 150A are mixed to provide whitelight.

Thus, the seventh embodiment allows one-chip integration of a lightemitting diode which provides white light by exciting the YAGfluorescent material contained in the red light emitting Eu doped layer150 and the insulating material 19 with light outputted from the bluelight emitting diode 10. Accordingly, the intensity of emission light inthe red range is higher in the semiconductor light emitting deviceaccording to the seventh embodiment than in the conventional white lightemitting diode which excites the YAG fluorescent material with lightoutputted from the blue light emitting diode. This allows white lighthaving an excellent color rendering property to be outputted.

It is also possible to provide the ultraviolet light emitting diode 30which outputs ultraviolet light at a wavelength of 340 nm in place ofthe blue light emitting diode 10 and excite each of the blue lightemitting fluorescent material, the green light emitting fluorescentmaterial, and the red light emitting Eu doped layer 150 with theultraviolet light to provide white light.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 24A through 24D and FIGS. 25A through 25C show the cross-sectionalstructures of the semiconductor light emitting device according to theseventh embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 24A, the n-type semiconductor layer 12 made ofr-type GaN, the active layer 13 having a multiple quantum well structuremade of InGaN, the p-type semiconductor layer 14 made of p-typeA_(0.05)Ga_(0.95)N, and an undoped Al_(0.5)Ga_(0.5)N layer 150B aregrown successively by MOCVD (Metal Organic Chemical Vapor Deposition) onthe substrate 11 made of sapphire having a principal surface of whichthe plane orientation is, e.g., the (0001) plane in the same manner asin the first embodiment. The n-type semiconductor layer 12, the activelayer 13, and the p-type semiconductor layer 14 emit blue light at awavelength of, e.g., 470 nm through current injection.

Next, as shown in FIG. 24B, europium (Eu) ions are implanted into thegrown Al_(0.5)Ga_(0.5)N layer 150B under such implant conditions that,e.g., an acceleration voltage is about 200 kV and the dose is about1×10¹⁵ cm⁻². Preferably, the substrate 11 is heated herein to about 500°C. during the implantation of Eu ions. Under these implant conditions,the concentration profile of Eu ions implanted in the Al_(0.5)Ga_(0.5)Nlayer 150B has a peak at a depth of 75 nm from the surface of theAl_(0.5)Ga_(0.5)N layer 150B. Subsequently to the ion implantation,annealing is performed in a nitrogen atmosphere at about 1000° C. forthe activation of the Eu ions, thereby obtaining the red light emittingEu doped layer 150 from the Al_(0.5)Ga_(0.5)N layer 150B. As describedabove, the red light emitting Eu doped layer 150 emits red light at awavelength of 622 nm through excitation caused by visible light orultraviolet light.

Next, as shown in FIG. 24C, the red light emitting Eu doped layer 150and an upper portion of the p-type semiconductor layer 14 areselectively removed by ICP etching using, e.g., a chlorine (Cl₂) gas byusing a metal mask (not shown) having a pattern consisting of aplurality of discrete and spaced-apart squares that has been placed onthe red light emitting Eu doped layer 150. At this stage, the portion ofthe red light emitting Eu doped layer 150 overlying the p-side electrodeformation region is also removed.

Next, as shown in FIG. 24D, the metal mask is removed and then thetransparent electrode 16 which is made of ITO with a thickness of about300 nm and transmits visible light is formed over the selectivelyexposed portions of the p-type semiconductor layer 14 and the red lightemitting Eu doped layer 150 that has been divided into a plurality ofislands each configured as a square when viewed in a plan view.Subsequently, the portion of the transparent electrode 16 overlying then-side electrode formation region is removed by wet etching using, e.g.,an aqueous hydrogen chloride (HCl) solution. Thereafter, a thermalprocess is performed in an oxygen atmosphere at a temperature of, e.g.about 600° C., thereby reducing the contact resistance and theresistivity of the transparent electrode 16 and improving thetransmittance thereof.

Next, as shown in FIG. 25A, the respective portions of the p-typesemiconductor layer 14 and the active layer 13 overlying the n-sideelectrode formation region 12 a are removed selectively by ICP etchingso that the n-side electrode formation region 12 a of the n-typesemiconductor layer 12 is exposed.

Next, as shown in FIG. 25B, the n-side electrode 18 as an ohmicelectrode composed of a multilayer film of titanium (Ti) and gold (Au)is formed on the exposed n-side electrode formation region 12 a of then-type semiconductor layer 12 by, e.g., sputtering. Thereafter,sintering may also be performed appropriately in a nitrogen atmosphereat a temperature of, e.g., about 550° C. to reduce the contactresistance of the n-side electrode 18. Subsequently, the p-sideelectrode 17 made of gold (Au) and serving as a p-side electrode pad isformed selectively by, e.g., sputtering on the p-side electrodeformation region of the transparent electrode 16.

Next, as shown in FIG. 25C, the resulting structure is divided intolight emitting diode chips each having a 350-μm square size by, e.g.,dicing. Subsequently, each of the chips resulting from the division ismounted on the specified region of the package 20 by using, e.g., asilver (Ag) paste. Thereafter, wire bonding is performed with respect tothe p-side electrode 17 and the n-side electrode 18 and the insulatingmaterial 19 containing a YAG fluorescent material is further applied tocover the chip.

Thus, the fabrication method according to the seventh embodiment allowsone-chip integration of a white light emitting diode in which the redlight emitting Eu doped layer 150 and the YAG fluorescent material areexcited by the blue light from the blue light emitting diode 10. Thisenables the intensity of emission light in the red range to be higherthan in the conventional white light emitting diode. As a result, itbecomes possible to provide a white light emitting diode which outputswhite light having an excellent color rendering property.

Although the seventh embodiment has doped the red light emitting Eudoped layer 150 with europium (Eu) as an element serving as aluminescent center, the doping element is not limited to Eu. Instead ofEu, samarium (Sm) or ytterbium (Yb) may also be used.

In each of the first through seventh embodiments, the plane orientationof the principal surface of sapphire used as the substrate for epitaxialgrowth is not particularly limited. In the case of using, e.g.,sapphire, a plane orientation at an off-angle from a representative(typical) plane orientation, such as the (0001) plane, may also beadopted.

The material of the substrate for epitaxial growth is not limited tosapphire. Besides sapphire, there can be used silicon carbide (SiC),zinc oxide (ZnO), silicon (Si), gallium arsenide (GaAs), galliumphosphide (GaP), indium phosphide (InP), gallium nitride (GaN), aluminumnitride (AlN), magnesium oxide (MgO), or lithium aluminum gallium oxide(LiAl_(x)Ga_(1-x)O₂ (where 0≦x≦1)).

The composition of each of the n-type semiconductor layers 12 and 32,the active layers 13 and 33, and the p-type semiconductor layers 14 and34 and the composition of the layer which emits light throughexcitation, such as the red light emitting layer 15, are not limited tothose shown in the individual embodiments. The method for crystal growthis not limited to MOCVD and a semiconductor layer formed by using, e.g.,an MBE (Molecular Beam Epitaxy) process or a HVPE (Hydride Vapor PhaseEpitaxy) process may also be contained. Each of the semiconductor layersmay also contain a group V element such as arsenide (As) or phosphorous(P) or a group III element such as boron (B).

Embodiment 8

An eighth embodiment of the present invention will be described withreference to the drawings.

FIG. 26A shows a cross-sectional structure of a semiconductor lightemitting device according to the eighth embodiment and FIG. 26B shows aplan structure thereof.

As shown in FIGS. 26A and 26B, the semiconductor light emitting deviceaccording to the eighth embodiment is composed of: a blue light emittingdiode 110 mounted at a specified position on a package 120 as a mountingmember; a red light emitting layer 116 grown epitaxially on the bluelight emitting diode 110; and an insulating material 119 containing anyttrium aluminum garnet (YAG) fluorescent material. In FIG. 26B, thedepiction of the insulating material 119 is omitted.

The blue light emitting diode 110 is composed of: an n-typesemiconductor layer 112 made of n-type GaN; an active layer 113 having amultiple quantum well structure made of InGaN; a p⁺-type semiconductorlayer 114 made of p⁺-type Al_(0.05)Ga_(0.95)N; and an n⁺-typesemiconductor layer 115 made of n⁺-type A_(0.05)Ga_(0.95)N which areepitaxially grown successively on a substrate 111 made of, e.g.,sapphire. The active layer 113 is composed of three well layers eachmade of In_(0.35)Ga_(0.65)N having a thickness of 2 nm and three barrierlayers each made of In_(0.02)Ga_(0.98)N having a thickness of 10 nmwhich are alternately stacked, thereby emitting blue light at 470 nm.

The red light emitting layer 116 is made of, e.g., undopedIn_(0.4)Ga_(0.6)N with a forbidden band width of 1.9 eV and formed onthe n⁺-type semiconductor layer 115 to have a matrix configurationconsisting of discrete and spaced-apart squares each with 2-1 μm to20-μm sides when viewed in a plan view. Each square of the red lightemitting layer 116 is excited by blue light outputted from the bluelight emitting diode 110 and emits red light at 650 nm. Red light can beobtained from the red light emitting layer 116 by doping the red lightemitting layer 15 with, e.g., zinc (Zn), magnesium (Mg), or silicon (Si)and thereby reducing the composition of In. By thus reducing thecomposition of In, a lattice mismatch with an underlie layer made of GaNnormally used can be suppressed and crystal defects in the red lightemitting layer 116 can be reduced so that high-brightness light emissionis enabled. At this time, the emission light released from the red lightemitting layer 116 is visible light generated via an energy levelresulting from the impurity used for doping and blue excitation light ismixed in the visible light.

Thus, in the semiconductor light emitting device according to the eighthembodiment, the n⁺-type semiconductor layer 115 is formed on the p⁺-typesemiconductor layer 114 of the blue light emitting diode 110 such thatthe p⁺-type semiconductor layer 114 and the n⁺-type semiconductor layer115 form a p⁺n⁺junction.

In the specification of the present application, a p⁺-type semiconductorlayer indicates a p-type semiconductor layer in which the concentrationof a p-type dopant, e.g., magnesium (Mg) is about 1×10²¹ cm⁻³ and ann⁺-type semiconductor layer indicates an n-type semiconductor layer inwhich the concentration of an n-type dopant, e.g., silicon (Si) is about1×10¹⁹ cm¹³.

The present invention can achieve an effect provided that the impurityconcentration in each of the p⁺-type semiconductor layer 114 and then⁺-type semiconductor layer 115 is 1×10¹⁸ cm³ or more.

When a voltage higher than the voltage applied to the n-typesemiconductor layer 112 is applied to the n⁺-type semiconductor layer115 between the p⁺-type semiconductor layer 114 and the n⁺-typesemiconductor layer 115, voltage-current characteristics close to alow-resistance ohmic characteristic are shown due to a tunnel currentgenerated in the p⁺n⁺ junction. Consequently, the same rectifyingproperty as provided by a normal pn junction is obtainable.

FIG. 27A shows the result of a comparison made between respectivecurrent-voltage characteristics when a tunnel junction (n⁺pn junction)is formed and when a normal pn junction is formed. FIG. 27B shows theresult of a comparison made between respective current-light outputcharacteristics when the tunnel junction (n⁺pn junction) is formed andwhen the normal pn junction is formed. As can be seen from FIG. 27A, anoperating voltage during the rising edge is higher than when the normalpn junction is formed. However, a light output is larger when the tunneljunction is formed provided that the injected current has the samevalue, as can be seen from FIG. 27B.

In the case of adopting the conventional structure in which the p-typesemiconductor layer is provided on the active layer, the transparentelectrode occupying a large area should normally be provided on thehigh-resistance p-type semiconductor layer, In the semiconductor lightemitting device according to the eighth embodiment, however, an injectedcurrent is sufficiently diffused even in a lateral direction (directionparallel to the substrate) without providing a transparent electrode onthe n⁺-type semiconductor layer 115 since the low-resistance n⁺-typesemiconductor layer 115 is provided on the p⁺-type semiconductor layer114. This obviates the necessity to provide a transparent electrode andallows respective ohmic electrodes provided in the n-type semiconductorlayer 112 and the n⁺-type semiconductor layer 115, which will bedescribed later, to have the same compositions. As a result, the n-typesemiconductor layer 112 and the n⁺-type semiconductor layer 115 can beformed by the same process and the fabrication process for thesemiconductor light emitting device can significantly be simplified.

A first ohmic n-side electrode 117 composed of a multilayer structure inwhich titanium (Ti), aluminum (Al), nickel (Ni), and gold (Au) films arestacked in layers is formed selectively on a region of the p⁺-typesemiconductor layer 115.

The n⁺-type semiconductor layer 112 has an exposed region and a secondohmic n-side electrode 118 composed of a multilayer structure in whichtitanium (Ti), aluminum (Al), nickel (Ni), and gold (Au) films arestacked in layers is formed on the exposed region. To the first andsecond n-side electrodes 117 and 118, required metal thin lines areconnected by wire bonding, though they are not depicted.

The insulating material 119 containing the YAG fluorescent material isformed as follows: The insulating material 119 is coated or applieddropwise onto the package 120 to cover the blue light emitting diode110, the red light emitting layer 116, the first n-side electrode 117,and the second n-side electrode 118 and then hardened. The insulatingmaterial 119 is excited by blue light outputted from the blue lightemitting diode 110 to emit yellow light.

A current is injected into the blue light emitting diode 110 via thefirst n-side electrode 117, the n⁺-type semiconductor layer 115, and thep-type semiconductor layer 114. Accordingly, white light having thespectrum shown in FIG. 28 can be obtained by injecting a current of,e.g., 20 mA into the blue light emitting diode 110 and thereby causingthe emission of blue light at a wavelength of 470 nm. In FIG. 28, theemission spectrum is composed of the transmitted component 110A of theblue light at a wavelength of 470 nm, yellow emission light 119A with apeak wavelength of 550 nm from the YAG fluorescent material, and the redlight 116A at a wavelength of 650 nm from the red light emitting layer116. The blue light 110A, the yellow light 119A, and the red light 116Aare mixed to provide white light.

Thus, the eighth embodiment allows one-chip integration of a lightemitting diode in which the red light emitting layer 116 which receivesblue light outputted from the blue light emitting diode 110 andgenerates red light through excitation caused thereby is providedbetween the insulating material 119 containing the YAG fluorescentmaterial which emits yellow light and the blue light emitting diode 110which emits blue light. Accordingly, the intensity of emission light inthe red range is higher than in an emission spectrum obtained from theconventional white light emitting diode shown in FIG. 37 which provideswhite light by exciting the YAG fluorescent material with blue lightfrom the blue light emitting diode. This allows a white light emittingdiode which outputs white light having an excellent color renderingproperty to be provided.

In addition, when an operating current is injected in the blue lightemitting diode 110, a so-called tunnel current flows in the p⁺n⁺junction formed between the p⁺-type semiconductor layer 114 and then⁺-type semiconductor layer 115 so that a rectifying property isobtained by applying a voltage between the n⁺-type semiconductor layer115 and the n-type semiconductor layer 112 such that it is higher to then⁺-type semiconductor layer 115 than to the n-type semiconductor layer112. Moreover, since the n⁺-type semiconductor layer 115 is low inresistance, the injected current is more likely to be diffused in thein-plane direction of the n⁺-type semiconductor layer 115. This obviatesthe necessity to use a transparent electrode as used conventionally andenhances the brightness of the emission light.

The blue light emitting diode 110 may also be formed with an underlielayer made of GaN and a thin-film buffer layer made of GaN or AlN beinginterposed between the substrate 111 made of sapphire and the n-typesemiconductor layer 112.

Although the eighth embodiment has patterned the red light emittinglayer 116 into islands each configured as a square when viewed in a planview, it need not necessarily be patterned. For example, the red lightemitting layer 116 may also be formed over the entire upper surface ofthe n⁺-type semiconductor layer 115 except for the region thereof to beformed with the first n-side electrode 117. In the case of forming thered light emitting layer 116 over the entire surface, the area of thered light emitting layer 116 is optimized to a value which optimizes thecolor rendering property of the output light.

The active layer 113 may also be constituted to have the composition ofIn which is nonuniform in the in-plane direction (direction parallel toa substrate surface) of the active layer 113.

Instead of varying a lattice constant in each of the n-typesemiconductor layer 112, the active layer 113, the p⁺-type semiconductorlayer 114, the n⁺-type semiconductor layer 115, and the readsemiconductor layer 116 which have been formed on the substrate 111, thecomposition of a group III element in a quaternary or higher-order mixedcrystal may also be varied in forming the blue light emitting diode 110and the red light emitting layer 116. This provides a structure fromwhich high-brightness light emission can be obtained without incurring acrystal defect due to a lattice mismatch and the resulting nonradiativerecombination.

In the eighth embodiment, the YAG fluorescent material and the red lightemitting layer 116 are excited by the output light received thereby fromthe blue light emitting diode 110 and emit yellow light and red light,respectively, thereby providing white light. However, an ultravioletlight emitting diode which output ultraviolet light at a wavelength of,e.g., 340 nm may also be formed in place of the blue light emittingdiode 110. In this case, a blue light emitting fluorescent material anda green light emitting fluorescent material are added to the insulatingmaterial 119.

It is also possible to separate the substrate 111 made of sapphire fromthe blue light emitting diode 110 and provide a metal film in place ofthe separated substrate. The arrangement allows the use of the providedmetal film as an n-side electrode and obviates the necessity to form then-side electrode 118 by exposing the n-type semiconductor layer 112.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 29A through 29E show the cross-sectional structures of thesemiconductor light emitting device according to the eighth embodimentin the individual process steps of the fabrication method therefor. Thedrawings show, of a wafer on which a plurality of semiconductor lightelements can be formed simultaneously, only one element formationregion.

First, as shown in FIG. 29A, the n-type semiconductor layer 112 made ofn-type GaN, the active layer 113 having a multiple quantum wellstructure made of InGaN, the p⁺-type semiconductor layer 114 made ofp⁺-type Al_(0.05)Ga_(0.95)N, the n⁺-type semiconductor layer 115 made ofn⁺-type Al_(0.05)Ga_(0.95)N, and the red light emitting layer 116 madeof undoped In_(0.4)Ga_(0.6)N are grown successively by MOCVD (MetalOrganic Chemical Vapor Deposition) on the substrate 111 made of sapphirehaving a principal surface of which the plane orientation is, e.g., the(0001) plane. As described above, the p⁺n⁺ junction formed between thep⁺-type semiconductor layer 114 and the n⁺-type semiconductor layer 115shows current-voltage characteristics close to a low-resistance ohmiccharacteristic due to the tunnel current. On the other hand, the activelayer 113 is composed of three quantum well layers each made ofIn_(0.35)Ga_(0.65)N having a thickness of 2 nm and three barrier layerseach made of In_(0.02)Ga_(0.98)N having a thickness of 10 mm, which arealternately stacked. However, the structure of the active layer 113 isnot limited thereto provided that the emission wavelength is about 470nm. It is possible to form an underlie layer made of GaN and a thin-filmbuffer layer made of GaN or AlN between the substrate 111 and the n-typesemiconductor layer 112. It is also possible to form the active layer113 such that the composition of In is nonuniform in the in-planedirection (direction parallel to the substrate surface) of the activelayer 113. It is also possible to obtain red emission light by usingInGaN doped with, e.g., zinc, magnesium, or silicon, instead of usingundoped In_(0.4)Ga_(0.6)N, and thereby forming the red light emittinglayer 116 such that the composition of In is lower than 0.4.

Next, as shown in FIG. 29B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 116. Subsequently, the red light emittinglayer 116 and an upper portion of the n⁺-type semiconductor layer 115are selectively removed by ICP (Inductive Coupled Plasma) etching using,e.g., a chlorine (Cl₂) gas by using the formed metal thin film as amask. At this stage, the portion of the red light emitting layer 116overlying the first n-side electrode formation region is also removed.

Next, as shown in FIG. 29C, the respective portions of the n⁺-typesemiconductor layer 115, the p⁺-type semiconductor layer 114, and theactive layer 113 overlying the second n-side electrode formation region112 a are selectively removed by ICP etching, whereby the second n-sideelectrode formation region 112 a of the n-type semiconductor layer 112is exposed.

Next, as shown in FIG. 29D, a resist film (not shown) having openingpatterns corresponding to the second n-side electrode formation region112 a and to the first n-side electrode formation region of the n⁺-typesemiconductor layer 115 is formed by lithography after dry etching.Subsequently, an electrode formation film composed of a multilayer filmof Ti, Al, Ni, and Au is formed by, e.g., electron beam vapor depositionover the entire surface of the formed resist film. Then, the first andsecond n-side electrodes 117 and 118, each of which is an ohmicelectrode, are formed simultaneously from the electrode formation filmby a so-called lift-off process which removes the resist film. To reducethe contact resistance of each of the first and second n-side electrodes117 and 118, sintering may also be performed appropriately in a nitrogenatmosphere at a temperature of, e.g., about 550° C.

Next, as shown in FIG. 29E, after the formation of the first and secondn-side electrodes 117 and 118, the resulting structure is divided intolight emitting diode chips each having a 350-μm square size by, e.g.,dicing. Subsequently, each of the chips resulting from the division ismounted on the specified region of the package 120 by using, e.g., asilver (Ag) paste. Thereafter, wire bonding is performed with respect tothe first and second n-side electrodes 117 and 118 and the insulatingmaterial 119 containing a YAG fluorescent material is further applied tocover the chip.

Thus, the fabrication method according to the eighth embodiment allowsone-chip integration of a white light emitting diode in which the redlight emitting layer 116 and the YAG fluorescent material are excited bythe blue light from the blue light emitting diode 110. This enables theintensity of emission light in the red range to be higher than in theconventional white light emitting diode. As a result, it becomespossible to provide a white light emitting diode which outputs whitelight having an excellent color rendering property.

In addition, since the n⁺-type semiconductor layer 115 made of a nitrideis lower in impurity energy level than a p-type semiconductor layer madeof a nitride or a p⁺-type semiconductor layer and therefore can bereduced in resistance, the diffusion of the current in a lateraldirection (direction parallel to the substrate surface) is sufficientlylarge so that a transparent electrode as shown in the conventionalembodiment need not be provided. This not only simplifies thefabrication process but also eliminates the absorption of the emissionlight by a transparent electrode, which occurs when the transparentelectrode is provided. As a result, a high-brightness white lightemitting diode can be provided.

Moreover, the first and second n-side electrodes 117 and 118, each ofwhich is an ohmic electrode, do not have different compositions but havethe same composition, in contrast to the p-side electrode and the n-sideelectrode having different compositions according to the conventionalembodiment. Accordingly, the first and second n-side electrodes 117 and118 can be formed simultaneously in the same process step. Thissimplifies the fabrication process and allows fabrication at a lowercost.

Although the eighth embodiment has adopted a structure in which theoutput light from the semiconductor light emitting device is extractedfrom the side with the n⁺-type semiconductor layer 115, so-calledflip-chip mounting may also be performed which forms a high-reflectivityelectrode composed of a multilayer film using, e.g., platinum (Pt),rhodium (Rh), or silver (Ag) for the lower layer thereof and using gold(Au) for the upper layer thereof on the red light emitting layer 116 andthe n⁺-type semiconductor layer 115 and mounts the high-reflectivityelectrode on the upper surface of the package 120 with a silver bump orthe like interposed between the high-reflectivity electrode and thepackage 120. If flip-chip mounting is performed, the output light passesthrough the substrate 111 made of sapphire to be extracted so that ahigh-brightness white light emitting diode is provided.

Embodiment 9

A ninth embodiment of the present invention will be described withreference to the drawings.

FIG. 30A shows a cross-sectional structure of a semiconductor lightemitting device according to the ninth embodiment and FIG. 30B shows aplan structure thereof. The description of the components shown in FIGS.30A and 30B which are the same as those shown in FIGS. 26A and 26B willbe omitted by retaining the same reference numerals.

As shown in FIGS. 30A and 30B, the semiconductor light emitting deviceaccording to the ninth embodiment is composed of: an ultraviolet lightemitting diode 130; a blue light emitting layer 121; a green lightemitting layer 122; and the red light emitting layer 116, each of whichhas been grown epitaxially on the ultraviolet light emitting diode 130.

The ninth embodiment is different from the eighth embodiment in that ithas provided the ultraviolet light emitting diode 130 as the lightemitting diode in place of the blue light emitting diode 110 and newlyprovided the blue light emitting layer 121 and the green light emittinglayer 122 which are excited by ultraviolet light at a wavelength of 340nm outputted from the ultraviolet light emitting diode 130 to emit bluelight at a wavelength of 470 n and green light at a wavelength of 555nm, respectively, thereby obviating the necessity for the insulatingmaterial 119 containing a YAG fluorescent material and covering theultraviolet light emitting diode 130.

The ultraviolet light emitting diode 130 is composed of: an n-typesemiconductor layer 132 made of, e.g., n-type Al_(0.1)Ga_(0.9)N; anactive layer 133 having a multiple quantum well structure made of InGaNand AlGaN; a p⁺-type semiconductor layer 134 made of p⁺-typeAl_(0.15)Ga_(0.85)N; and an n⁺-type semiconductor layer 135 made ofn⁺-type Al_(0.15)Ga_(0.85)N. The active layer 133 is composed of fivewell layers each made of In_(0.02)Ga_(0.98)N having a thickness of 1.5nm and five barrier layers each made of Al_(0.15)Ga_(0.85)N having athickness of 10 nm which are alternately stacked, thereby emittingultraviolet light at 340 nm.

The blue light emitting layer 121 is made of, e.g., undopedIn_(0.15)Ga_(0.85)N with a forbidden band width of 2.6 eV and formed onthe n⁺-type semiconductor layer 135 to have a configuration consistingof a plurality of discrete and spaced-apart islands. The green lightemitting layer 122 is made of, e.g., undoped In_(0.2)Ga_(0.8)N with aforbidden band width of 2.2 eV and formed on the blue light emittinglayer 121 to have the same plan configuration as the blue light emittinglayer 121. The red light emitting layer 116 is made of, e.g., undopedIn_(0.4)Ga_(0.6)N with a forbidden band width of 1.9 eV and formed onthe green light emitting layer 122 to have the same plan configurationas the green light emitting layer 122. The blue light emitting layer121, the green light emitting layer 122, and the red light emittinglayer 116 are excited by the ultraviolet light outputted from theultraviolet light emitting diode 130 to emit blue light at a wavelengthof 470 nm, green light at a wavelength of 555 nm, and red light at awavelength of 650 nm. Blue light emission, green light emission, and redlight emission can be obtained from the blue light emitting layer 121,the green light emitting layer 122, and the red tight emitting layer 116by doping each of the blue light emitting layer 121, the green lightemitting layer 122, and the red light emitting layer 116 with, e.g.,zinc (Zn), magnesium (Mg), or silicon (Si) and thereby reducing thecomposition of In in each of the light emitting layers. By thus reducingthe composition of In, a lattice mismatch with an underlie layer made ofGaN normally used can be suppressed and crystal defects in the bluelight emitting layer 121, in the green light emitting layer 122, and inthe red light emitting layer 116 can be reduced so that high-brightnesslight emission is enabled.

If a voltage is applied between the n-type semiconductor layer 132 ofthe ultraviolet light emitting diode 130 and the n⁺-type semiconductorlayer 135 thereof to be higher to the n⁺-type semiconductor layer 135than to the n-type semiconductor layer 132, voltage-currentcharacteristics close to a low-resistance ohmic characteristic areobserved due to a tunnel current occurring in the p⁺n⁺ junction, asdescribed in the eighth embodiment, so that the same rectifying propertyas obtained with a normal pn junction is obtainable. In addition, theinjected current is diffused sufficiently even in a lateral direction(direction parallel to the substrate) without providing a transparentelectrode on the n⁺-type semiconductor layer 135, the first n-sideelectrode 117 provided on the n⁺-type semiconductor layer 135 and thesecond n-side electrode 118 provided on the n-type semiconductor layer132 can be formed to have the same composition, as will be describedlater. This allows the first n-side electrode and the second n-sideelectrode 118 to be formed in one process step and remarkably simplifiesa fabrication process for the semiconductor light emitting device.

A current is injected into the ultraviolet light emitting diode 130 viathe first n-side electrode 117, the n⁺-type semiconductor layer 135, andthe p⁺-type semiconductor layer 134. Accordingly, white light having thespectrum shown in FIG. 31 can be obtained by injecting a current of,e.g., 20 mA into the ultraviolet light emitting diode 130 and therebycausing the emission of ultraviolet light at a wavelength of 340 nm. InFIG. 31, the emission spectrum is composed of the transmitted component130A of the ultraviolet light at a wavelength of 340 nm which is low inintensity, blue light 121A with a peak wavelength of 470 nm from theblue light emitting layer 121, green light 122A with a peak wavelengthof 555 nm from the green light emitting layer 122, and the red light116A at a wavelength of 650 nm from the red light emitting layer 116.The blue light 121A, the green light 122A, and the red light 116A aremixed to provide white light.

Thus, the ninth embodiment allows one-chip integration of a lightemitting diode in which the blue light emitting layer 121, the greenlight emitting layer 122, and the red light emitting layer 116 whichreceive ultraviolet light outputted from the ultraviolet light emittingdiode 130 and generate blue light, green light, and red light throughexcitation caused by the received ultraviolet light are provided on theultraviolet light emitting diode 130. Accordingly, the intensity ofemission light in the red range is higher than in an emission spectrumobtained from the conventional white light emitting diode which provideswhite light by exciting the YAG fluorescent material with blue lightfrom the blue light emitting diode. This allows a white light emittingdiode which outputs white light having an excellent color renderingproperty to be provided.

In addition, when an operating current is injected in the ultravioletlight emitting diode 130, a so-called tunnel current flows in the p⁺n⁺junction formed between the p⁺-type semiconductor layer 134 and then⁺-type semiconductor layer 135 so that a rectifying property isobtained by applying a voltage between the n⁺-type semiconductor layer135 and the n-type semiconductor layer 132 such that it is higher to then⁺-type semiconductor layer 135 than to the n-type semiconductor layer132. Moreover, since the n⁺-type semiconductor layer 135 is low inimpurity energy level and can be reduced in resistance, the injectedcurrent is more likely to be diffused in the in-plane direction of then-type semiconductor layer 135. This obviates the necessity to use atransparent electrode as used conventionally and enhances the brightnessof the emission light. Moreover, since an insulating material containinga YAG fluorescent material is unnecessary, the steps of forming thetransparent electrode and the insulating material containing the YAGfluorescent material can be omitted so that the fabrication process issimplified.

For easy transmission of the excitation light, the thickness of each ofthe red light emitting layer 116 and the green light emitting layer 122is minimized provided that sufficient emission light is obtained.

The ultraviolet light emitting diode 130 may also be formed with anunderlie layer made of GaN and a thin-film buffer layer made of GaN orAlN being interposed between the substrate 111 made of sapphire and then-type semiconductor layer 132.

Although the ninth embodiment has patterned each of the blue lightemitting layer 121, the green light emitting layer 122, and the redlight emitting layer 116 into islands each configured as a square whenviewed in a plan view, they need not necessarily be patterned. Forexample, each of the blue light emitting layer 121, the green lightemitting layer 122, and the red light emitting layer 116 may also beformed over the entire upper surface of the n⁺-type semiconductor layer135 except for the region thereof to be formed with the first n-sideelectrode 117. In the case of forming each of the light emitting layers121, 122, and 116 over the entire surface, the area of the lightemitting layer is optimized to a value which optimizes the colorrendering property of the output light.

In the case where the ultraviolet light emitting diode 130 is replacedwith the blue light emitting diode 110 used in the eighth embodiment,the blue light emitting layer 121 need not be provided.

It is also possible to separate the substrate 111 made of sapphire fromthe ultraviolet light emitting diode 130 and provide a metal film inplace of the separated substrate. The arrangement allows the providedmetal film to be used as an n-side electrode and obviates the necessityto form the second n-side electrode 118 by exposing the n-typesemiconductor layer 132.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 32A through 32D show the cross-sectional structures of thesemiconductor light emitting device according to the ninth embodiment inthe individual process steps of the fabrication method therefor.

First, as shown in FIG. 32A, the n-type semiconductor layer 132 made ofn-type GaN, the active layer 133 having a multiple quantum wellstructure made of InGaN, the p⁺-type semiconductor layer 134 made ofp⁺-type Al_(0.05)Ga_(0.95)N, the n⁺-type semiconductor layer 135 made ofn⁺-type Al_(0.05)Ga_(0.95)N, the blue light emitting layer 121 made ofundoped In_(0.15)Ga_(0.85)N, the green light emitting layer 122 made ofundoped In_(0.2)Ga_(0.8)N, and the red light emitting layer 116 made ofundoped In_(0.4)Ga_(0.6)N are grown successively by MOCVD on thesubstrate 111 made of sapphire having a principal surface of which theplane orientation is, e.g., the (0001) plane. As stated previously, thep⁺n⁺ junction formed between the p⁺-type semiconductor layer 134 and then⁺-type semiconductor layer 135 shows voltage-current characteristicsclose to a low-resistance ohmic characteristic due to the tunnelcurrent. The active layer 133 is composed of five quantum well layerseach made of In_(0.02)Ga_(0.85)N having a thickness of 1.5 nm and fivebarrier layers each made of Al_(0.15)Ga_(0.85)N having a thickness of 10nm, which are alternately stacked. However, the structure of the activelayer 133 is not limited thereto provided that the emission wavelengthis about 340 nm. The forbidden band width of the blue light emittinglayer 121 is 2.6 eV and emits blue light at 470 nm. The forbidden bandwidth of the green light emitting layer 122 is 2.3 eV and emits greenlight at 555 nm. The forbidden band width of the red light emittinglayer 116 is 1.9 eV and emits red light at 650 nm. It is possible toform an underlie layer made of GaN and a thin-film buffer layer made ofGaN or AlN between the substrate 111 and the n-type semiconductor layer132. The active layer 133 may also be constituted to have thecomposition of In which is nonuniform in the in-plane direction(direction parallel to a substrate surface) of the active layer 133. Itis also possible to obtain red emission light by using InGaN doped with,e.g., zinc, magnesium, or silicon, instead of using undopedIn_(0.4)Ga_(0.6)N, and thereby forming the red light emitting layer 116such that the composition of In is lower than 0.4.

Next, as shown in FIG. 32B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 116. Subsequently, respective upperportions of the red light emitting layer 116, the green light emittinglayer 122, the blue light emitting layer 121, and the n⁺-typesemiconductor layer 135 are selectively removed by ICP etching using,e.g., a chlorine (Cl₂) gas by using the formed metal thin film as amask. At this stage, the respective portions of the red light emittinglayer 116, the green light emitting layer 122, and the blue lightemitting layer 121 overlying the first n-side electrode formation regionare also removed.

Next, as shown in FIG. 32C, the respective portions of the n⁺-typesemiconductor layer 135, the p⁺-type semiconductor layer 134, and theactive layer 133 overlying the second n-side electrode formation layer132 a are selectively removed by ICP etching, whereby the n-sideelectrode formation layer 132 a of the n-type semiconductor layer 132 isexposed.

Next, as shown in FIG. 32D, a resist film (not shown) having openingpatterns corresponding to the second n-side electrode formation layer132 a and to the first n-side electrode formation region of the n⁺-typesemiconductor layer 135 is formed by lithography after dry etching.Subsequently, an electrode formation film composed of a multilayer filmof Ti, Al, Ni, and Au is formed by, e.g., electron beam vapor depositionover the entire surface of the formed resist film. Then, the first andsecond n-side electrodes 117 and 118, each of which is an ohmicelectrode, are formed from the electrode formation film by a so-calledlift-off process which removes the resist film. To reduce the contactresistance of each of the first and second n-side electrodes 117 and118, sintering may also be performed appropriately in a nitrogenatmosphere at a temperature of, e.g., about 550° C. Subsequently, theresulting structure is divided into light emitting diode chips eachhaving a 350-μm square size by, e.g., dicing. Further, each of the chipsresulting from the division is mounted on the specified region of thepackage 120 by using, e.g., a silver (Ag) paste. Thereafter, wirebonding is performed with respect to the first and second n-sideelectrodes 117 and 118.

Thus, the fabrication method according to the ninth embodiment allowsone-chip integration of a white light emitting diode in which the bluelight emitting layer 121, the green light emitting layer 122, and thered light emitting layer 116 are excited by the ultraviolet light fromthe ultraviolet light emitting diode 130. This enables the intensity ofemission light in the red range to be higher than in the conventionalwhite light emitting diode. As a result, it becomes possible to providea white light emitting diode which outputs white light having anexcellent color rendering property.

In addition, since the n⁺-type semiconductor layer 135 made of a nitrideis lower in impurity energy level than a p-type semiconductor layer madeof a nitride or a p⁺-type semiconductor layer and therefore can bereduced in resistance, the diffusion of the current in a lateraldirection (direction parallel to the substrate surface) is sufficientlylarge so that a transparent electrode as shown in the conventionalembodiment need not be provided. This not only simplifies thefabrication process but also eliminates the absorption of the emissionlight by a transparent electrode, which occurs when the transparentelectrode is provided. As a result, a high-brightness white lightemitting diode can be provided.

In the fabrication method according to the ninth embodiment, the step ofproviding an insulating material containing a YAG fluorescent materialis unnecessary. In addition, since the first and second n-sideelectrodes 117 and 118, each of which is an ohmic electrode, have thesame composition, they can be formed in the same process step. Thisfurther simplifies the fabrication process and allows fabrication atlower cost.

In the ninth embodiment also, it is possible to form an electrode madeof a high-reflectivity material as described in the eighth embodimentand perform flip-chip mounting which mounts the formed electrode made ofthe high-reflectivity material with a silver bump or the like interposedbetween the package and the electrode.

Embodiment 10

A tenth embodiment of the present invention will be described withreference to the drawings.

FIG. 33A shows a cross-sectional structure of a semiconductor lightemitting device according to the tenth embodiment and FIG. 33B shows aplan structure thereof. The description of the components shown in FIGS.33A and 33B which are the same as those shown in FIGS. 30A and 30B willbe omitted by retaining the same reference numerals.

The tenth embodiment is different from the ninth embodiment in that asubstrate made of sapphire is separated and removed from the ultravioletlight emitting diode 130 during the epitaxial growth thereof, the secondn-side electrode 118 composed of a multilayer structure of titanium (Ti)and gold (Au) is provided on the surface of the n-type semiconductorlayer 132 from which the substrate has been removed, and a plating layer131 made of gold (Au) with a thickness of 10 μm or more, e.g., about 50μm is provided to substantially function as the n-side electrode.Preferably, a material having a reflectivity of 60% or more with respectto ultraviolet light, blue light, green light, and red light is usedherein for the second n-side electrode 118. For example, a single layerfilm made of, e.g., gold (Au), platinum (Pt), copper (Cu), silver (Ag),or rhodium (Rh) or a multilayer film containing at least two of theforegoing elements can be used. For the gold plating layer 131, copper(Cu) or silver (Ag) can be used instead of gold (Au).

If ultraviolet light at a wavelength of 340 μm is generated by injectinga current of, e.g., 20 mA in the ultraviolet light emitting diode 130,white light having a spectrum pattern equal to the spectrum shown inFIG. 31 is obtained.

With the arrangement, the semiconductor light emitting device accordingto the tenth embodiment can be integrated as one chip in which the bluelight emitting layer 121, the green light emitting layer 122, and thered light emitting layer 116 which receive ultraviolet light outputtedfrom the ultraviolet light emitting diode 130 and generate blue light,green light, and red light through excitation caused by the receivedultraviolet light are provided on the ultraviolet light emitting diode130, similarly to the semiconductor light emitting device according tothe ninth embodiment. Accordingly, the intensity of emission light inthe red range is higher than in an emission spectrum obtained from theconventional white light emitting diode which provides white light byexciting the YAG fluorescent material with blue light from the bluelight emitting diode. This allows a white light emitting diode whichoutputs white light having an excellent color rendering property to beprovided.

Since it is unnecessary to provide an insulating material containing aYAG fluorescent material and a transparent electrode as providedconventionally, the absorption of emission light by the transparentelectrode, which occurs when the transparent electrode is provided, isno more observed so that higher brightness is achieved.

In addition, the tenth embodiment has formed the second n-side electrode118 over the entire surface (back surface) of the n-type semiconductorlayer 132 opposite to the active layer 133 by removing the substratemade of sapphire for epitaxial growth and further provided the goldplating layer 131 with a relatively large thickness of 50 μm. Thearrangement significantly improves the heat dissipation property of theultraviolet light emitting diode 130 and allows a higher-output whitelight emitting diode to be provided. In addition, since the secondn-side electrode 118 and the first n-side electrode 117 are disposed inopposing relation with the active layer 133 interposed therebetween, aseries resistance between the second n-side electrode 118 and the firstn-side electrode 117 can be reduced advantageously. Since the insulatingsubstrate made of sapphire or the like has been removed, it isunnecessary to provide the second n-side electrode 118 on the upperportion of the n-type semiconductor layer 132. This achieves a reductionin chip size and allows the elimination of the step of etching away then-side semiconductor layer 132 from the side of the p⁺-typesemiconductor layer 134.

For easy transmission of the excitation light, the thickness of each ofthe red light emitting layer 116 and the green light emitting layer 122is minimized provided that sufficient emission light is obtained.

The ultraviolet light emitting diode 130 may also be formed with anunderlie layer made of GaN and a thin-film buffer layer made of GaN orAlN being interposed between the substrate 111 made of sapphire and then-type semiconductor layer 132.

Although the tenth embodiment has patterned each of the blue lightemitting layer 121, the green light emitting layer 122, and the redlight emitting layer 116 into islands each configured as a square whenviewed in a plan view, they need not necessarily be patterned. Forexample, the blue light emitting layer 121, the green light emittinglayer 122, and the red light emitting layer 116 may also be formed overthe entire upper surface of the n⁺-type semiconductor layer 135 exceptfor the region thereof to be formed with the first n-side electrode 117.In the case of forming each of the light emitting layers 121, 122, and116 over the entire surface, the area of the light emitting layer isoptimized to a value which optimizes the color rendering property of theoutput light.

In the case where the ultraviolet light emitting diode 130 is replacedwith the blue light emitting diode 110 used in the eighth embodiment,the blue light emitting layer 121 need not be provided.

Referring to the drawings, a description will be given herein below to amethod for fabricating the semiconductor light emitting device thusconstructed.

FIGS. 34A through 34D and FIGS. 35A through 35C show the cross-sectionalstructures of the semiconductor light emitting device according to thetenth embodiment in the individual process steps of the fabricationmethod therefor.

First, as shown in FIG. 34A, the n-type semiconductor layer 132 made ofn-type GaN, the active layer 133 having a multiple quantum wellstructure made of InGaN, the p⁺-type semiconductor layer 134 made ofp⁺-type Al_(0.05)Ga_(0.95)N, the n⁺-type semiconductor layer 135 made ofn⁺-type Al_(0.05)Ga_(0.95)N, the blue light emitting layer 121 made ofundoped In_(0.15)Ga_(0.85)N, the green light emitting layer 122 made ofundoped In_(0.2)Ga_(0.8)N, and the red light emitting layer 116 made ofundoped In_(0.4)Ga_(0.6)N are grown successively by MO CVD on thesubstrate 111 made of sapphire having, a principal surface of which theplane orientation is, e.g., the (0001) plane. As stated previously, thep⁺n⁺ junction formed between the p⁺-type semiconductor layer 134 and then⁺-type semiconductor layer 135 shows voltage-current characteristicsclose to a low-resistance ohmic characteristic due to the tunnelcurrent. The active layer 133 is composed of five quantum well layerseach made of In_(0.02)Ga_(0.98)N having a thickness of 1.5 nm and fivebarrier layers each made of Al_(0.15)Ga_(0.85)N having a thickness of 10nm, which are alternately stacked. However, the structure of the activelayer 133 is not limited thereto provided that the emission wavelengthis about 340 nm. The forbidden band width of the blue light emittinglayer 121 is 2.6 eV and emits blue light at 470 mm. The forbidden bandwidth of the green light emitting layer 122 is 2.3 eV and emits greenright at 555 nm. The forbidden band width of the red light emittinglayer 116 is 1.9 eV and emits red light at 650 mm. It is possible toform an underlie layer made of GaN and a thin-film buffer layer made ofGaN or AlN between the substrate 111 and the n-type semiconductor layer132. The active layer 133 may also be constituted to have thecomposition of In which is nonuniform in the in-plane direction(direction parallel to a substrate surface) of the active layer 133. Itis also possible to obtain red emission light by using InGaN doped with,e.g., zinc, magnesium, or silicon, instead of using undopedIn_(0.4)Ga_(0.6)N, and thereby forming the red light emitting layer 116such that the composition of In is lower than 0.4.

Next, as shown in FIG. 34B, a metal thin film (not shown) made of nickeland having a pattern consisting of a plurality of discrete andspaced-apart squares each having, e.g., 2-μm to 20-μm sides is formed onthe red light emitting layer 116. Subsequently, respective upperportions of the red light emitting layer 116, the green light emittinglayer 122, the blue light emitting layer 121, and the n⁺-typesemiconductor layer 135 are selectively removed by ICP etching using,e.g., a chlorine (Cl₂) gas by using the formed metal thin film as amask. At this stage, the respective portions of the red light emittinglayer 116, the green light emitting layer 122, and the blue lightemitting layer 121 overlying the first n-side electrode formation regionare also removed.

Next, as shown in FIG. 34C, a resist film (not shown) having an openingpattern corresponding to the first n-side electrode formation region ofthe n⁺-type semiconductor layer 135 is formed by lithography after dryetching. Subsequently, an electrode formation film composed of amultilayer film of Ti, Al, Ni, and Au is formed by, e.g., electron beamvapor deposition over the entire surface of the formed resist film.Then, the first n-side electrode 117, which is an ohmic electrode, isformed from the electrode formation film by a so-called lift-off processwhich removes the resist film. To reduce the contact resistance of thefirst n-side electrode 117, sintering may also be performedappropriately in a nitrogen atmosphere at a temperature of, e.g., about550° C.

Next, as shown in FIG. 34D, after the formation of the first p-sideelectrode 117, a holding substrate 151 made of silicon is bonded to then⁺-type semiconductor layer 135 containing the first n-side electrode117, the red light emitting layer 116, and the like by using, e.g., anepoxy-based adhesive agent 152. The material of the holding substrate151 is not limited to silicon. A polymer film may also be used for theholding substrate 151.

Next, as shown in FIG. 35A, a high-output and short-wavelength pulselaser beam which is not absorbed by the substrate 111 but is absorbed bythe n-type semiconductor layer 132, such as the third-harmonic beam of aYAG laser at a wavelength of 355 m or a KrF excimer laser beam at awavelength of 248 nm, is applied in a scanning manner to the surface ofthe substrate 111 opposite to the holding substrate 151 for theirradiation thereof. At this time, the applied laser beam is absorbed bythe portion of the n-type semiconductor layer 132 made of GaN which isadjacent to the interface between itself and the substrate 111. As aresult, the portion of the n-type semiconductor layer 132 which isadjacent to the interface with the substrate 111 is heated and, if thetemperature becomes 900° C. or higher through the absorption of thelaser beam, the portion of the n-type semiconductor layer 132 adjacentto the interface with the substrate 111 is decomposed into a metalgallium (Ga) gas and a nitrogen (N₂) gas, so that a decomposition layeris formed.

Then, the substrate 111 formed with the decomposition layer is separatedfrom the n-type semiconductor layer 132 by heating the substrate 111 toa temperature not less than 29° C., which is a melting point of gallium,or by immersing the substrate 111 in an aqueous hydrogen chloride (HCl)solution and thereby melting or removing metal gallium contained in thedecomposition layer. Thereafter, the second n-side electrode 118composed of a multilayer film of titanium (Ti) and gold (Au) is formedby, e.g., electron beam vapor deposition on the exposed surface fromwhich the substrate 111 has been separated and removed. Subsequently,the gold plating layer 131 with a thickness of about 50 μm is formed byelectrolytic plating using the gold (Au) layer of the second n-sideelectrode 118 as an underlie, whereby the structure shown in FIG. 35B isobtained.

Next, as shown in FIG. 35C, the adhesive agent 53 is removed by using,e.g., acetone so that the holding substrate 151 is removed. Then, theresulting structure is divided into light emitting diode chips eachhaving a 350-μm square size by, e.g., dicing. Subsequently, each of thechips resulting from the division is mounted on the specified region ofa package (not shown) by using, e.g., a silver (Ag) paste. Thereafter,wire bonding is performed with respect to the first n-side electrode117, whereby the white light emitting diode is obtained.

Thus, the fabrication method according to the tenth embodiment allowsone-chip integration of a white light emitting diode in which each ofthe blue light emitting layer 121, the green light emitting layer 122,and the red light emitting layer 116 is excited by the ultraviolet lightoutputted from the ultraviolet light emitting diode 130. This enablesthe intensity of emission light in the red range to be higher than inthe conventional white light emitting diode. As a result, it becomespossible to provide a white light emitting diode outputs white lighthaving an excellent color rendering property.

In addition, since the n⁺-type semiconductor layer 135 made of a nitrideis lower in impurity energy level than a p-type semiconductor layer madeof a nitride or a p⁺-type semiconductor layer and therefore can bereduced in resistance, the diffusion of the current in a lateraldirection (direction parallel to the substrate surface) is sufficientlylarge so that a transparent electrode as shown in the conventionalembodiment need not be provided. This not only simplifies thefabrication process but also eliminates the absorption of the emissionlight by a transparent electrode, which occurs when the transparentelectrode is provided. As a result, a high-brightness white lightemitting diode can be provided.

In addition, the substrate made of sapphire which is not excellent inheat dissipation property is removed and the gold plating layer 131which is excellent in heat dissipation property is provided in placethereof so that a higher output is produced.

Although the tenth embodiment has described the separation method whichapplies the high-output short-wavelength pulse laser beam for theseparation of the substrate 111 made of sapphire, the separation of thesubstrate 111 is not limited to the method using laser irradiation. Itis also possible to, e.g., use the substrate 111 made of silicon (Si) orgallium arsenide (GaAs) in place of the substrate 111 made of sapphireand separate and remove the substrate 111 by wet etching using an acid.

In each of the ninth and tenth embodiments, the plane orientation of theprincipal surface of sapphire used as the substrate for epitaxial growthis not particularly limited. In the case of using, e.g., sapphire, aplane orientation at an off-angle from a representative (typical) planeorientation, such as the (0001) plane, may also be adopted.

The material of the substrate for epitaxial growth is not limited tosapphire. Besides sapphire, there can be used silicon carbide (SiC),zinc oxide (ZnO), silicon (Si), gallium arsenide (GaAs), galliumphosphide (GaP), indium phosphide (InP), gallium nitride (GaN), aluminumnitride (AlN), magnesium oxide (MgO), or lithium aluminum gallium oxide(LiAl_(x)Ga_(1-x)O₂ (where 0≦x≦1)).

The composition of each of the nitride semiconductor layers composingthe blue light emitting diode 110 and the ultraviolet light emittingdiode 130 and the composition of each of the nitride semiconductorlayers composing the individual layers which emit light throughexcitation, such as the red light emitting layer 116, are not limited tothose shown in the individual embodiments. The method for crystal growthis not limited to MOCVD and a semiconductor layer formed by using, e.g.,an MBE (Molecular Beam Epitaxy) process or a HVPE (Hydride Vapor PhaseEpitaxy) process may also be contained. Each of the semiconductor layersmay also contain a group V element such as arsenide (As) or phosphorous(P) or a group III element such as boron (B) as a constituent element.

As described above, the semiconductor light emitting device according tothe present invention allows one-chip integration of a high-output whitelight emitting diode structure featuring an excellent color renderingproperty and high brightness and is useful as the back light of a liquidcrystal display device, a white light source for illumination, or thelike.

1. A semiconductor light emitting device comprising: a light emittingdiode comprising a plurality of semiconductor layers for emitting firstemission light; a semiconductor film provided to absorb the firstemission light and emit second emission light; and a transparentelectrode provided in the light emitting diode, wherein thesemiconductor film emits the second emission light through opticalexcitation by the first emission light, wherein the transparentelectrode transmits the first emission light or the second emissionlight, wherein the semiconductor film and the transparent electrode arelocated on a light emitting surface of the light emitting diode, and andwherein the transparent electrode is provided with a plurality ofopenings and the semiconductor film is located in each of the openingson a light emitting surface of the light emitting diode.
 2. Thesemiconductor light emitting device of claim 1, further comprising: afluorescent material covering the light emitting diode and thesemiconductor film, wherein the fluorescent material absorbs the firstemission light and emits third emission light.
 3. The semiconductorlight emitting device of claim 1, wherein the light emitting diode orthe semiconductor film is formed on a substrate made of a singlecrystal.
 4. The semiconductor light emitting device of claim 3, whereinthe substrate transmits the first emission light and the second emissionlight.
 5. The semiconductor light emitting device of claim 3, whereinthe single crystal is sapphire, silicon carbide, gallium nitride,aluminum nitride, magnesium oxide, lithium gallium oxide, lithiumaluminum oxide, lithium aluminum oxide, or a mixed crystal of lithiumgallium oxide and lithium aluminum oxide.
 6. The semiconductor lightemitting device of claim 1, wherein the semiconductor film is formed tocover a part of the transparent electrode in the vicinity of theopenings.
 7. The semiconductor light emitting device of claim 6, whereinthe semiconductor film has a crystal defect density which is lower inthe portion thereof located on the transparent electrode than in each ofthe portions thereof located over the individual openings of thetransparent electrode.
 8. The semiconductor light emitting device ofclaim 1, wherein the first emission light is blue light or ultravioletlight.
 9. The semiconductor light emitting device of claim 8, whereinthe semiconductor film is excited by the first emission light to emitthe second emission light which is red light.
 10. The semiconductorlight emitting device of claim 1, wherein the semiconductor film iscomposed of a plurality of semiconductor films which are stacked inlayers and emit emission light components having different wavelengthsfrom each of the stacked layers.
 11. The semiconductor light emittingdevice of claim 1, further comprising a metal film in a portion of theelectrode and having a thickness of at least 10 μm, wherein a current isinjected in the light emitting diode through the metal film.
 12. Thesemiconductor light emitting device of claim 11, wherein the metal filmis made of gold, copper, or silver.
 13. The semiconductor light emittingdevice of claim 1, wherein the light emitting diode is provided with ametal electrode having a reflectivity of 60% or more with respect to thefirst emission light or the second emission light.
 14. The semiconductorlight emitting device of claim 13, wherein the metal electrode iscomposed of a single-layer film or a multi-layer film each made of atleast one material selected from the group consisting of gold, platinum,copper, silver, and rhodium.